A primer on Pretreatment DPD/DPYD testing: Dr. Thomas Holden from Stanford Cancer Institute (2026)

Manju George
Hello everyone. Welcome to DocTalks. I’m Dr. Manju George, the Scientific Director at PALTOWN Development Foundation, the nonprofit that supports colontown. It’s my pleasure to welcome Dr. Thomas Holden. He’s a board certified, fellowship trained medical oncologist and a clinical assistant professor at the Stanford University School of Medicine.

Since the updated black box warning, FDA warning on capecitabine and 5 FU, there have been many posts in Colontown asking about DPD testing, and Dr. Holden had given this fantastic talk at ASCO GI, earlier this year in January. And when I reached out to him requesting a Doc Talk, he was very generous.

And so we really appreciate your time and your willingness to educate colorectal cancer patients and caregivers on this topic. Welcome Dr. Holden.

Dr. Thomas Holden
Thanks so much. Thank you so much for the opportunity to come give a talk today on something that I think is really been a hot topic over the last year.

And not just for people with colon and rectal cancer, although that I know that’s the main audience here today. But for anyone with a cancer diagnosis that warrants treatment with 5 FU capecitabine, I think this would be important. My name is Thomas Holden. I’m a clinical assistant professor at Stanford.

I’m a medical oncologist who specializes in GI oncology. And today I’m gonna give you a primer on pre-treatment dihydro pyrimidine dehydrogenase, also known as DPD or DPYD testing, which we’ll talk a lot more about. Just a brief slide to orient us on key terminology. These terms are confusing, and so when we go through slides or lectures or you hear about this, you’re gonna hear the word DPD, DPYD people throwing ’em around.

I’m gonna be honest with you. And upfront, there are many doctors who get the terms confused. But to be clear, the DPYD gene, the four letter, that is the gene, and that codes for the enzyme in your body. Dihydro pyrimidine dehydrogenase, which we abbreviate as DPD, so DPD with the three letters is the enzyme, DPYD, which is usually italicized is the gene.

There are some, a couple takeaways for the talk, that I wanna make sure we really highlight. Number one. DPYD testing, testing for the gene is now recommended for all patients before starting treatment with 5 fluorouracil, which we abbreviate as 5 FU or capecitabine therapy. Number two, there is guidance that exists for both the testing, how to order testing, and what to do with the results, how to manage those tests.

And then three, I’ll talk a little bit about at the end, about how we need to build broader systems in the United States and not rely on individual doctors talking to their patients and relying at the individual level. We need to have a broader scheme in place to make sure this testing is done appropriately and there’s guidance.

So we’re gonna do a little bit of updates over the past year. Okay. So the beginning of 2025, over a year ago now, there was a safety announcement from the FDA. They highlighted the importance of DPD deficiency discussions with all patients before starting these treatments. So they advise that healthcare providers, doctors have a discussion with patients, about the risks of toxicity, the risks, and the need to do testing or consider testing before you start these therapies. Now this is something that was not to my knowledge and to my experience in talking with other providers and patients, had not been aggressively adopted over 2025 amongst all providers.

Why? Because it was a recommendation. They said, consider doing this. And this fell on the heels of an important meeting where the huge stakeholders in the FDA met with the American Association for Cancer Research, the AACR, where there were the greatest thought leaders. They reviewed all of the data, all the information about the toxicity of these medications, about where we stood with the technology for testing for DPD deficiency and what came from that was a slew of announcements over this past year. Fast forward to October of 2025. The drug label for capecitabine written here, you’ll see in all capitals Xeloda, that’s the trade name for capecitabine. Those are the same. So the FDA updated their drug label for capecitabine in October of 2025. And the big change here was they said that prior to initiating this drug, you should test patients for genetic variants of DPYD. Notice how they didn’t say consider testing or have a discussion with your patient about testing.

They say there is a risk of serious toxicity, you should test all patients. Okay, now within 30 days of that FDA label update, the NCCN, the National Comprehensive Cancer Network, the colon panel, which is some of the greatest thought leaders in colon cancer who run that panel, they met and they updated their guideline.

They acknowledged that the FDA had made this change and incorporated, and the NCCN not only acknowledged, but they incorporated it into their recommendations to test. And then they also made the comment that although the Infusional 5 fluorouracil label was not yet updated at that time. It should apply to patients receiving 5 fluorouracil as well as capecitabine.

This is a big deal because, for doctors it’s a bit understood in the world of oncology, but for patients, they may not realize that. Doctors really are afraid to do any changes to their practice unless the NCCN tells them to do it. The doctors use it as their blueprint for making treatment decisions for almost anything involving cancer.

It’s our predominant blueprint. Fast forward a little bit further this year, February 5th, 2026, the FDA also updated the label to 5 fluorouracil. So now the label was updated for both drugs, okay, to say, to test now how I’ve here it again in red to test. Patients for genetic variants of the gene, DPYD prior to initiating treatment.

You’ll notice here at the end, they leave a comment. They say, unless immediate treatment is necessary, the understanding thought here is that yes, for some patients in certain situations, we cannot wait. They urgently need to start these treatments and we say, well, of course we can of course send the test, but we have a risk benefit discussion and sometimes treatment must be started and we cannot wait now.

I wanna do a brief review because we’ve reviewed kind of the updates over the last year, but why now? Why in 2025, 2026, over the last 20 years we’ve been using these drugs? Let’s review these drugs a little bit more. So on the left we have the molecule, 5 fluorouracil. On the right we have the molecule capecitabine.

I want to just put them side by side because I think it’s really important to remind both doctors and patients that these drugs, if you look at what’s highlighted– in my red box here on the right compared to 5 fluorouracil on the left– these drugs, functionally they end up acting the exact same in your body.

Capecitabine is a pro drug to 5 fluorouracil So your body takes a few extra steps, but it breaks it into 5 fluorouracil. So these drugs, in terms of their side effects, their toxicity and their effect against cancer, they are in effect the same, they’re anti metabolite pyrimidine analogs. So what does that mean?

It means once they’re in your body and get incorporated inside the cancer cells, they inhibit DNA and they damage RNA in rapidly dividing cells. So they work very well. That’s why we still use them today. They work very well in killing rapidly dividing cancer cells, particularly in colon and rectal cancer.

A final point. They are used ubiquitously across colon and rectal cancer. It does not matter what stage of colon or rectal cancer you have stage two, stage three, stage four. In almost all cases, with some rare exceptions, you would be recommended to get one of these drugs.

Now let’s review some important features about the enzyme dihydro pyrimidine dehydrogenase. Everyone who doesn’t have a severe deficiency has this enzyme in their body, in all of their cells. So this graphic here is meant to teach a couple of points. One is that if you look at the area in green on this picture this is inside of the cell. And you can see here it mentions the green circle, the 5 FU as it goes into the cell– one of the immediate rate limiting breakdown steps or catabolic steps is on the left here you see highlighted in red. This box, DPD, this is the enzyme. It metabolizes 5 FU immediately to a harmless metabolite called dihydro fluorouracil DHFU. And the rest of the pathway is for its active effects against cancer. Now, if you are deficient in DPD, which I’ve highlighted in red here, you would start getting an excess or buildup of active metabolites that could cause toxicity. So I see here you have FDUMP, FUTP and FDUTP. So these could lead to what we call off-target toxicity in higher amounts and effectively harming normal cells, where normally if you have adequate function of DPD, you would not be exposed to that excess risk of toxicity.

It’s important to note that again when these drugs are given to you, this enzyme breaks down more than 80% of this drug inside the cell, and it’s the remainder that’s having its active effect against the cancer cells. Now, if you do have a deficiency of the enzyme, that’s significant. What are you at risk?

What is this off-target toxicity that clinicians need to be monitoring for and severely worried about? There is severe neutropenia and frankly with severe deficiency, pancytopenia, which means all of your blood cell lines can be decreased, but neutropenia is one that we worry about the most.

You can have severe diarrhea, you can have severe mucositis, the mucus membranes in your mouth and elsewhere. Hand foot syndrome. So your, the skin of your hands and your feet and even other parts of your body can begin to peel and slough off. You can have neurologic toxicity, other end organ failure, and with severe deficiency, you can have death.

It’s important to note that with testing, if you identify someone who has a deficiency, then a lot of these can be prevented. For some of these toxicity is preventable if you know in advance and you do the testing and it’s important to highlight that the risk of death overall in people who have DPD deficiency, we can’t understate it.

The risk is there. There was a systematic review that was done in 2021 that looked at the most well-defined gene variants in the literature and the risk of death associated and the overall conclusion, looking at all these data was that the treatment related mortality, which is the risk of death when someone’s on treatment is 2.3% compared to people who do not

have a gene, a germline variant. And carriers of pathogenic DPYD gene variants have an overall 25.6 times increased risk of treatment related death. Now, of course, I want to be clear that the risk of death is still exceptionally low, even in patients across the board who have deficiency, but the amplitude and the increased risk of death is real.

Now, I do want to take a moment also and talk about we do have a way to try to reverse sudden severe toxicity that’s been approved since 2015 in the United States. Uridine Tri Acetate, it’s also known as Vista Guard. It’s an analog that helps bind and outcompete the 5 fluorouracil and capecitabine. And it’s approved for people who have overdosed. So they take more than was prescribed or more than was intended by accident or if they exhibit early onset– so within 96 hours– severe life-threatening side effects, like we’ve already talked about. And they looked at giving this drug within the first few days compared to

historic controls where nothing was done and they just support. And what we see is that the risk of death is significantly decreased if you immediately can administer this rescue agent. Now, most hospitals do have a way to obtain this agent, and so this is something that we have to watch out for. And before people were getting DPYD testing and we necessarily didn’t know sometimes a decision has to be made very quickly on the fly to administer this agent not knowing if someone is truly deficient or not, just based on symptoms alone. So we’ve reviewed kind of the the current status quo, okay? Which is now, as of the last year with these updates from the FDA, there is a formal recommendation that before starting these treatments, we should be testing everybody.

That is the recommendation, that is the guidance based on all of the literature that’s been reviewed. So now we need to talk about the guidance. What testing should we do? What kind of tests should we order, and what do we do with the results? How do we interpret them, and how do we let that guide our decisions?

So as a clinician, when I see that the FDA has this blanket statement where they say, we need to test people for DPD deficiency. So the first question to me that comes to mind as a doctor is what test should I order? So I need guidance there. What do the test results mean? How do I interpret the results that come to my desk as a physician?

And then how do I use that result to guide treatment, to give patients the best treatment available based on that information that is now in front of me? So a big table here, but I want you to focus on a couple things. For what test should we order? The Association for Molecular Pathology has done great work over these past years to help define tiers of what we call must-test variants.

So there are, frankly, hundreds of types of mutations that can happen to the DPYD gene, but there are ones that are, we are getting more and more familiar with understanding which ones are the baddest actors. So we look at a couple of things when we try to define this tier list, this way to categorize the worst mutations out there of all of them.

We look at mutations that result in loss of function of the enzyme, or at least measurably decreased function, and with that we associate an activity value. We’ll go into more of that later. It’s a little bit, we’ll explain it a bit more, but that’s one part. And then the other thing that we look at when we define these tiers– how common are they, not just in Caucasians, but how common are they across multi-ethnic populations? How often do we see these? Because we need to take both of these into consideration when we’re offering a test that’s trying to test and make sure we capture the baddest actors, the worst mutations that cause actual, clinically significant loss of function of the enzyme.

So then where does that leave us now? Well, this table here, it’s not definitive. So every month, as access to testing and more companies are offered more and more expanding testing as time goes on, this will change. This is kind of a snapshot of where we stand in the last few months of commercially available tests and which ones test at least all of the tier one variants, like I said, the baddest actors and at least test some if not all, of the tier two variants. And then some even go beyond that and test other much more rare variants as well. But a couple things to reiterate is that the access and the coverage of this, so pharmacogenomic testing is improving by the month, by the year so this will get better and better. And right now, again, looking at this, there are commercially available many tests that are available now. It’s true that certain oncology clinics, if you’re not part of a huge academic center, they may still and for a while, yet may not have the ability in their lab to test for this.

They might have to send this– draw a tube of blood– and they might have to send it out to one of these companies to run, but they have the ability to do that. Something else that I wanna spend a little bit more time explaining as well is that even if your doctor does not send for a specific DPYD test, they may get a result by when they’re checking for something else, specifically next generation sequencing or molecular testing. Your doctor may talk to you about this because for a lot of patients with colon and rectal cancer, even for stage three and even sometimes for stage two, they may want to immediately test molecular testing. And now for molecular testing, liquid platforms such as Caris, Guardant, Tempus, Foundation– is updating this as well. They will automatically, with their next generation sequencing molecular blood tests, they will check this for you. So the doctor will get the result and you will get the result whether you specifically were looking for it or not.

And they do this automatically for most of the platforms now. In case anyone was curious, because access is expanding over time, rapidly and new tests are being developed and the amount of coverage of each individual platform expands over time. The NIH has a genetic testing registry where these tests are submitted and updated. So the link is on this page for anyone who’s curious.

And for any doctor who says, I wanna look and find a test that has the most expanded coverage possible, you can manually go and search. You go to the NIH Genetic Testing registry website. You type in DPYD, and it will give you a slew of institutions and organizations who test for DPYD.

And so it’s always good to be able to stay updated that way as well. So, we’ve talked about availability of testing and how it can be done. This slide here is giving you an example of this is what a doctor, and a patient may see on a test report, either with a molecular test like we talked about through a Caris, a Guardant, or a Tempus or Foundation or a specific DP YD test that shows up in their portal, their online health portal.

And it may show a result, this is an example of an abnormal result or someone who has deficiency in the enzyme, and there’s a lot of things on here, and we’ll go over this in subsequent slides and go over and explain why these things are on here. But when you have a result, admittedly it is confusing and even for doctors who are newly getting used to testing this, they may see these results and say, okay, I need to sit down and, and figure out what really this means.

So that’s what we’re going to do. But first the way I like to overall conceptualize what the test results mean is, I like to compartmentalize things into buckets. Because at the end of the day, when you get a test result back, what you, what the doctor needs to know to be able to make a decision about dosing or not giving a medicine is you need to know if they have complete deficiency, meaning your enzyme is not doing anything and you’re not able to metabolize that drug, 5 FU or capecitabine. Do you have partial deficiency where the enzyme is working to a degree, but you need to probably make a change in the dose of the treatment to avoid serious side effects, or do they have no deficiency, they have regular function of the enzyme, in which case there really needs to be no consideration of reducing treatment doses to start. I highlight the one in the middle because this is where I think doctors will get the most confused. They want to have specific guidance based on how much function the patient’s enzyme is actually working in their body. They want to know, okay, what should I do in this particular case? Because not all people who are partial deficient– you’re not, it’s not created equal. Some people are more partially deficient than others based on the mutation that they have in the gene.

And partial deficiency is common. 6% of the population has probably some mutation in DPYD, leading to some amount of deficiency of their enzyme function. So when you talk about six out of every 100 patients, that’s not insignificant. So. I’m gonna highlight a few things on this table because this is our like nomenclature, when we look at a test report, what we are actually looking for, to make a treatment decision.

And there’s a couple things. First and the end thing we wanna know is we want to know the resultant phenotype. What is the end result of having a deficiency in the enzyme and their resultant expression? We have people who are normal metabolizers, intermediate and poor. Associated with that phenotype is a reported activity score where zero means there is no activity of your DPD enzyme in your body.

An activity score of two means normal activity, meaning your enzyme is able to break down these drugs, the enzyme in your body is able to break it down without much concern. But there is varying levels in between, as you see here, there is 0.5 to 1.5, and those are varying degrees of intermediate and poor metabolizer where we have to have special considerations.

And of course the test needs to let us know what genotype is responsible for that activity score and that phenotype. So. When we have that report back, we see the resultant activity score and their phenotype of what type of metabolizer a patient is. Well, how do we actually make a decision on dosing?

Thankfully, there is a guiding body and organization, the Clinical Pharmaceutical Implementation Consortium, known as CPIC, they established a working guideline in 2017 and are updating it regularly with another update scheduled later this year. They look at the resultant activity score phenotype and provide initial dosing recommendations based off of that. So again, you see here with activity scores ranging down below from zero to two. They give a recommendation of standard dosing. And as much as we would love for that to be a one size fits all, I wanna point out something. If you look at the DPYD activity score of 1.5, you notice that they say reduce the starting dose by 25 to 50%.

So what’s the controversy there? Well, as a clinician, when a doctor sees this, doctors want to be told in these types of cases, they want to be told exactly what to do by the guidelines. And when you see this range of reduce their drug by 25 to 50%, what are doctors worried about?

They’re worried that if I reduce your drug by 50% instead of 25%, what if I’m causing harm? What if I’m not treating the cancer as aggressively as we should because I lowered the dose more than we need it to? So that’s what, that is a holdup that doctors think about and we have to consider very importantly, okay, so it’s not necessarily just because of this activity score of one glove fits all.

So how do we reconcile this? Well, I wanna say that these things are now being looked at prospectively. So there’s good news, we are looking at this and, and we will get more information and CPIC will update their guidelines accordingly as we start getting more data on this. Because now more and more people are getting tested and we know what their gene mutation is and what their deficiency is, and we can prospectively follow them– meaning going forward in time, their outcomes, and make changes to treatment recommendations based on that. So these graphs here, I’m going to explain to you, this is an example of a prospective study where they followed patients who they knew had a mutation in DPYD and decreased enzyme activity, and they followed the CPIC guidelines to dose reduce them.

So you see a graph on the top. They have a particular mutation in DPYD, the gene. They started with a 50% dose reduction and they did not make any further changes to the dose. They just cut their dose in half and they followed them longitudinally in time. And this is a graph looking at survival, or we call progression-free survival.

And you’ll see that the graphs pretty much overlap. Meaning that even though they reduced the dose by 50%, the outcomes of these patients, the survival, how long they did well over time– they were very close, so they didn’t think there was a significant difference, and so that dose reduction was very appropriate and they minimized excessive toxicity.

But if we look at a different mutation down below where they had an activity score of 1.5, they dose reduced the patients by only 25% and they followed them in time. And what you’ll see here is compared to patients who didn’t have a mutation, okay, and got the full dose, and what they see here is the graph separates a little bit.

Meaning that the people who had the gene mutation, they had slightly worse survival. And they also had slightly still some more toxicity. So this is an example, even lowering the dose, we think that their outcomes may be worse just by lowering the dose by 25%. So how do we, what do we do with this? How do we reconcile this?

The answer is there needs to be close monitoring for side effects and early dose modifications based on those side effects. What I mean is if someone is reduced their dose of capecitabine or 5 FU and they are tolerating it just fine at that reduced dose, clinicians should very closely, in follow up ,consider escalating the dose of their medicine to get them to the most effective and best tolerated dose possible.

So it’s not something that we start them on a dose reduction or reduce the dose of the drug and start a patient on a drug, then we don’t have the ability to increase the dose. If they tolerate it just fine, it is unlikely that they’re going to have a life-threatening side effect

if you increase the dose stepwise as time goes on. That way, you’re trying to get them the most effective dose of the drug possible. So this table kind of consolidates those recommendations and those considerations. So depending on the activity score and this DPYD allele mutation, so for example, HAP3 here on the top and star 2 A at the bottom. You start with the CPIC recommended dose reduction, and then the critical thing here is that you increase the dose. If they tolerate it well in a stepwise fashion, you gradually try to get them to the most effective dose in the most tolerable way possible and consider, we would call dose escalation.

I wanna pivot a little bit because it’s important to have a talk, and this is a very busy slide, but I’m gonna break it down for you. It’s important to talk about people who are not Caucasian, it’s like non-European and non- Caucasian. Because most of the data historically, when we look at the baddest actors, the worst gene variants are derived from European (Caucasian) patients.

But there has been extensive systematic review now within the last five years, with these changing recommendations to look at patients who are not Caucasian, to look at and see if we can identify other gene mutations that may be contributing to toxicity. So this systematic review in 2024 looked back at patients with case studies, retrospective review.

They took any data they could find to find patients, and this is in this review, they found 53 different mutations in the gene across patients who were not Caucasian. And they were all, they’re all listed here on this nice little infographic. The reason I put this here is to highlight a few points.

One is that if you look at the ones that are, and it might be a little small to see, but the ones that are bolded and blue, there’s only four of them. Those are the classic worst mutations that we saw historically in Europeans. And so certainly those are still present in people who are, non-Caucasian.

There are other ones listed here that are highlighted in blue. And these are the classical — the American Association for Pathology– they list as tier one and some tier two variants, so ones that are well described, but as you can see here, there’s a lot that are listed only in black that are probably far more rare and not nearly as common, but maybe associated with toxicity from these drugs.

And so this is just an example. In a graphical way and in a good way to show at least in a picture that there’s a lot of work to be done to make sure we are identifying still all the mutations possible that could be affecting everyone regardless of their race. So now, there’s a few additional clinical pearls I wanna highlight. A negative test, with we’ve talked about the testing that’s available now, and a lot of these tests, tests for, like we said, the worst actors. But if you undergo a test and you get a negative test result, there is still that rare possibility that you have an extremely rare variant that has not been identified in the literature that could explain some amount of toxicity that you have to these medications.

And on that note, having a DPYD variant does not guarantee actually that you will have toxicity. So there is this fear, and we talk about these dose reductions, but you have to think about historically, for the last 20 years when we weren’t testing anybody and they were getting the full dose of the drug. Most people, we think that retrospectively, we would say that like now we know, 6% of all these people had some type of variant, and a lot of them did, probably did okay even with the full dose. So there’s not a guarantee that you’ll have severe toxicity, but enough that as I said, the FDA has made their call, they’ve made their statement, we should be checking everybody.

And in the absence of a variant,– so let’s say you get testing and you have a normal test result. You still can have toxicity from these drugs, and it may, it’s possible that you have a variant that was not checked for, a very rare variant that was not checked for in the test.

But just because you don’t have a defined variant does not mean that these drugs are gonna be a walk in the park. Unfortunately, we still, even in people who have normal function, because you have to understand these drugs, the doses that we give them at in people, even without a variant in DPYD, these drugs were approved at what we call the maximum tolerated dose.

We give these drugs at a highest dose possible to be as aggressive against the cancer as possible. And so side effects are still going to be possible. Now, I wanted to spend some time on a few key points at the end because we’ve talked about that testing is recommended. We have guidelines that exist on what kind of tests we should order.

How should we should make management decisions based on those results? But now we need to focus on how we pave the way forward, because in the United States is perhaps a little bit late to the game. Anyone watching from Europe may know that well, I, I’ve been doing this since 2021, because they have, and anyone who is a veteran, who’s in the VA they’ve actually been checking this for a little bit of time now, before the rest of the US has gotten on board.

But it’s not enough to just have these guidelines floating around. We need to build automated systems and not rely on individual doctors to have to make these decisions in a busy clinic. We need to have this automated so that it’s taken away the pressure from both the patient and the doctor of what to do about this.

Call to action for system-wide protocols. Doctors and healthcare centers, cancer centers, clinics need to have electronic medical record, computer order integration in pharmacy workflows. We need to be able to order chemotherapy and have this test pop up and say, Hey, you’re about to start someone on capecitabine in 5fluorouracil. You should order this DPYD test. We’re gonna auto-populate that into your order set so that when you start and sign all of your orders for your patient, that will be there. If there needs to be a pre-treatment warning to say, Hey, oh, you’re about to sign an order for someone’s capecitabine or their 5fluorouracil

And the computer should probably, should have a flag that pops up and says, we don’t see a DPYD test result. Why is that? Can you please acknowledge? Now, of course, there may be reasons why treatment is needed before that is checked. That’s totally fine. But there should be an acknowledgement from the doctor before he treats someone to say, oh yeah, we acknowledge this is why.

And there needs to be institution and clinic wide, and that may vary a little bit from clinic or institution, but to have dose adjustments institutionalized at that level so that, again, a doctor is not having to make that decision on the fly and perhaps busy, tired day, they’re not quite looking at it all.

It needs to be very much in an algorithm type setting where that onus is taken off of the individual. I also wanna do a call to action for clinical trials. Many of you may be part of a clinical trial. Many of you may be considered for one in the future. Historically a lot of these clinical trials, if they involve you getting an investigational agent, there’s a lot of talk around right now, and I’m a clinical investigator myself around KRAS drugs, KRAS G12D, KRAS G12C.

These are very common in colon and rectal cancer. 40% of colon and rectal cancer patients have a mutation that may be promising for a clinical trial, and a lot of these trials exclude anyone with known DDPD deficiency historically because one, it wasn’t checked very often. Why do they do this? They’re worried, right? They’re worried that if someone has a deficiency and they go on their trial, what if they get really sick if we give them 5FU or capecitabine with an experimental drug? And my call to action, it’s to really everyone in the healthcare community and people who run and design clinical trials that we are now risking, excluding another 6% of the entire population if they have a partial deficiency in the enzyme.

So we need to consider modifying clinical trial protocols to make sure that we say we’re only excluding people who don’t have functional enzyme. If that trial is gonna give them capecitabine or 5FU.. Let people who have partial deficiency if we have guidelines to safely give them these drugs, let them get on the clinical trial and we can monitor that safely.

And not only that, we can stratify them on the study, perform correlative pharmacokinetics, check their blood levels, maximize both safety and efficacy, and follow their outcomes. Do patients who have a DPYD deficiency, do they do better? Do they do worse on this clinical trial,– stratify and we follow that.

So it’s my call to action that we continue to try to say that, and we’re continuing to work on in the healthcare setting. And then a call to action to the healthcare community as a whole for people who are not Caucasian– DPYD testing is important regardless of ethnicity, in my opinion.

It is true that you will see there are abstracts that have been published to say, well, for certain populations, certain ethnic populations, they don’t have any of these baddest actor, gene mutations, that’s fine and good, but the fact of the matter is they may have ones that are far more rare.

It’s true. Equally devastating. Clinically, it may be possible. So we are getting into an age where regardless of race or ethnicity, pharmacogenomic testing is going to become so good that perhaps, and my hope is with a single pin prick, we’ll be able to map the entire gene on everybody before they start any treatment for any cancer.

It’s not just in cancer treatment, but in all of healthcare, pharmacogenomic testing will be able to find all of this out. We’re not there yet, but we’re working on it. So in the meantime, we need to understand that testing panels have limitations. Like we talked about earlier, we are testing for and continuing to expand and testing more and more variants.

But there’s that possibility that we are missing some of the rarer ones. Still, even if people come back with a normal test, it may not be the most definitive test and we’re working on it. We need to focus on close clinical monitoring of every patient. Regardless of their race or regardless of their genotype result.

So even if they have a normal test result, it doesn’t change what doctors do. Historically, even before we were testing everyone, all clinicians monitor for the toxicity of these drugs, we are all well versed in it. We know what to look out for. And it’s true, people with deficiency of the enzyme have a much higher risk of getting these toxicities and they are preventable with the testing.

We need no matter what, regardless of the test result, close clinical monitoring. And to reiterate again, if they’re tolerating it at a dose reduction, we try to increase their dose to make sure it’s as tolerable as possible. And with that. I wanna give a thank you again to everyone allowing me to come and talk to you today.

I wanna give a special shout out to two paragons of the field who are probably the most published on this, and they’re wonderful individuals. Daniel Hertz, University of Michigan, and Alan Venook, who is a professor of oncology at UCSF.

Manju George
Thank you. Yeah. Thank you so much. That was amazing. You covered so much and really, really well.

You have a gift of breaking things down and explaining it so well. Thank you. If you have questions, you can use the Q and A. Please send your questions. I’ll start. Dr. Holden, can you go through, some clinical scenarios? Like, let’s say you’ve got a new patient, so can you tell me what exactly you do?

Dr. Thomas Holden
Yeah. So, currently our workflow at Stanford is if we have a new patient. Of course, I treat a lot of colon and rectal cancer. I treat anal cancer, I treat, other cancers well, gastric, pancreas. It depends on the type of cancer, but anyone, there is even a shadow of a doubt that we are going to give them capecitabine or 5FU. I usually, the day that I meet them in clinic, I have them stop at the lab. And right now Stanford’s current workflow is we do a send out DPYD gene test that checks all the tier one and two variants. It’s basically the full genotype, to the best of their ability.

It’s one of the most extensive commercially available tests, and we send it out to the Mayo Clinic. The turnaround time is about three to five business days in my experience, which usually is okay because when we meet someone for the first time, it takes us about two weeks before we execute any chemotherapy plan because they need, a lot of times they may need additional imaging, they may need a port, but we would do that.

And so that’s my current workflow and that’s what I’ve helped work to make sure that all of the oncologists who are treating patients with 5 fluorouracil or capecitabine, I’ve made sure that’s built in. So it’s built into our electronic medical record that when you order capecitabine or  5 fluorouracil the test comes down.

It says, okay, would you like to order it right now?

Manju George
So you’re saying that even before anything, that’s the first thing you do. Then you also talked about there are companies like Caris and Foundation Medicine and Guardant. So how are you doing that?

Dr. Thomas Holden
Yeah, that, it’s a good question. So, as far as indications for those, it’s true that not everyone, even though like we

do it for a lot of colon, a lot of rectal cancer, their cancers maybe not at a stage where we would do that testing. For example, a lot of providers for stage two, in stage one cancers, they would never do the molecular testing. But for anyone who is, in my opinion, in my recommendation, anyone who has a stage three or stage four colon or rectal cancer, they should undergo what we call NGS molecular testing with one of these companies.

And usually, because we do the DPYD, the lab test very quickly I like to do the tissue molecular testing, but it, sometimes tissue is an issue, so to speak. So someone, their doctor says, well, we would’ve sent this test, but the biopsy that we did, there just wasn’t enough tissue there.

That’s

when absolutely, these blood tests that all of these companies offer, and they all check, by the way, they all check for DPYD, they do pharmacogenomic testing too, and it’s no additional charge to the patient. It’s part of their huge panel. So, I highly encourage, anyone with stage three or stage four to get it, even if tissue is an issue or not.

That sounds good. I just want to say that like on Colontown University, we have a CRC 1 0 1 module that we have, in that we have this treatment toxicity testing is what we have called it. Because pharmacogenomics, many people might not know it. And then we explained, so Dan Hertz, we met him at ASCO GI, and then we had him look over the material that we had. And he was very gracious and he gave us comments. So this, your talk will be part of that,

and we are really excited.

And like I said, you have a gift for explaining it in a very

systematic way, which is wonderful.

Yeah, I think that this is one point that I wanted to highlight again, is that this is confusing for doctors. There are a lot of medical oncologists still, even with these recommendations, like again, they get DPD versus DPYD. It’s confusing. It, it’s incredibly confusing. And you have to sit down with it for a little while and we have to rely on the companies and the people who do the testing to be able to show it to us in a very digestible way.  But even still, um, any way I can help to break it down, yeah.

Okay. Okay. I’m not seeing any questions. Let me think about any other thing. So in your talk, you were talking about European and Caucasian. So by that you mean the ancestry, right? Not where you’re living.

Correct, correct. Of Ancestry. That’s absolutely correct. Yes.

Yeah. Okay. And then, what other thing? So let’s say you have a patient and you know, like what you were saying, they, maybe they were already on in treatment, right? They were, let’s say, it’s a stage four patient and who’s on FOLFOX for some time, and then now they come to know about the testing. Um, would you consider testing them now?

That’s a really good question. I love that question. So we are running into that now. The answer is if someone had been on treatment with these drugs for quite some time. And they’ve had no issues. I don’t feel strongly about it. Why? Because with the toxicity and the enzyme deficiency, we will know from the very first dose if they’re having toxicity from an enzyme deficiency.

Why? Because I mean, this enzyme, like we said, it breaks down 80% of this drug in your body. And so we know immediately. And if someone’s been on these drugs for months and months and they’ve had like only some minor issues here and there, and I say minor, but like things that are tolerable, I don’t feel strongly about it.

It’s of course more information, more knowledge. A lot of times we’ll get it. Anyways, because like I said, now with the molecular testing, the molecular platforms, they just automatically test it. So I’ve had a lot of cases where, for another reason, I’m, I’m checking for new mutations. Oh, do they have, , a new mutation after being on treatment for a year and a half to see if there’s anything else we can go after.

And then I happen to get the test result back. They’re like, they’re deficient in DPYD. And I’m like, I guess that explains why their platelets went down a little bit more like this past three months than I would’ve expected. But it doesn’t, it wouldn’t necessarily change my management at that point.

It’s all clinical management because they’ve been doing overall well.

Manju George
Okay. Okay. And then I also want you to address, because we have seen in COLONTOWN, I think we had a little bit of a discussion with you before we started the webinar– that there are patients who see the discussion about DPD testing in COLONTOWN, and they’re like, oh, my doctor hasn’t ordered it. Why is that? I’d like you to say something– I think you mentioned it saying it’s not widespread yet, right?

Dr. Thomas Holden
Yeah. So this is, again, to reiterate. It’s only October of 2025 that the NCCN even on just one cancer panel, the colon cancer panel, changed that recommendation. So I’ll have to double check the last time I checked, I don’t know if the rectal cancer panel, I don’t know if the anal cancer panel has added it to the NCCN yet. It unfortunately, those updates take time. And like I mentioned, doctors, unless it’s on there, they don’t necessarily immediately change their practice unless they see it there. And I think another part of it is that it’s been this topic, although it’s been a huge wave of a hot topic over the last year, has been going on and off for two decades. And I, I think because of that, some doctors like out there in the community who’ve been practicing for a long time, they’ll say, okay, is it for real this time?

Are we really doing this? There’s because we’ve been doing fine, for so long, and, and that’s the thing I wanna highlight is that we are talking about a very small number of patients. And doctors, of course are so comfortable managing these drugs as is and they know to immediately, if someone has a toxicity, what we’re really worried about is, and why the change now is that harm for that very small number of people who really, they’re, they don’t have the enzyme function. So it will adopt. I think this is going to be it. I think this is the final push where all doctors are going to gradually, hopefully over within, over the next year, they will adopt it. Because the big thing here is the FDA has said to test on the label for both drugs. And so now we have no recourse. We should test. And so I think if people need to just prepare, all the doctors are going to come around in the next few months.

Manju George
Okay. Okay. Thank you so much for that. My next question is could you also explain when you say complete deficiency versus partial could you go through what are the signs of a complete deficiency?

Dr. Thomas Holden
You mean, the clinical signs, so, I think unfortunately, complete deficiency is so rare that, I think I can count on one hand the number of cases I’ve been in involved with. They’re rare, they are. Complete deficiency cases are profound. You might have heard stories of people who they suddenly, they start 5 fluorouracil, so they start capecitabine.. And then within, days to no later than a week after starting, or within two weeks of starting for the capecitabine pills, profound symptoms. Like I said, horrible mucositis. Organ failure. They’re immediately put in the hospital, usually go to the ICU and they’re on supportive care. In some cases where they suspect it, doctors will immediately provide, like I said, the Vista Guard, the reversal agent to try to fix what’s going on.

But it’s profound. Complete deficiency is profound, and severe partial deficiency where they have like just a little enzyme is also, it can be very, very profound. In unfortunately those cases, it’s like once you recognize it. It’s too late. And that is, again, one of the impetuses of like, why we need to test before someone, we should be checking before someone starts because although it’s exceptionally rare, it’s too late once they’ve get the drug. The toxicity is so rapid and it’s so severe. Just to provide a little bit of reassure if someone starts treatment and they go through their first cycle of treatment and they have some significant side effects, but then they recover and they get better.

That is not a true– I would not worry. In those cases, their enzyme is working enough in their body. They don’t have a complete loss or deficiency. The complete loss or deficiency, they start the drug and immediately afterwards, it’s a life-threatening situation.

Manju George
Okay, so we have had some patients where they are in their first or second cycle and they are so severely dehydrated. They’re in the hospital, they’re getting fluids. So would you think that those are people with partial deficiency? We have not a lot. Like every year there’ll be some patients who are like, I’m in the hospital, I don’t know what to do. I have severe diarrhea, — I’ve not heard so much about mucositis– diarrhea and severe dehydration. That’s what I’ve heard.

Dr. Thomas Holden
Exactly. So those symptoms are.. We can suspect it if it’s bad enough. Like, in my experience, I find the capecitabine pills, Xeloda pills are a little bit. Infusional 5 FU with the diarrhea. Sometimes it’s a little bit of a nuance. But we do see it. And of course in my mind, in my cases for my patients, we’re testing before we start treatment. But those are cases where absolutely it should be brought up to say like, oh my gosh, I was in the hospital for a week with diarrhea. I was dehydrated, I was getting IV fluids. It should be checked. Absolutely. Just to make sure. Now classically, when we talked about the other things like mucositis, organ failure, these other things that can happen. The neutropenia, usually if the enzyme is severely deficient, we will see a lot of those things hand in hand. It’s usually not just the diarrhea, like all of the parts, all of the healthy parts of your body are somewhat affected when you have that severe deficiency.

So it’s kind of like a, we call like a constitutional toxicity. But it’s still possible, so absolutely. You should check if you have any severe, like one of the side effects that we should, you should make sure we, we have that so we know.

Manju George
Okay, so my next question is that we also have some patients who have coronary vasospasms. So people, they’ve got their first 5FU and they have chest pain. Do you think that that is something that is also– they don’t have the other diarrhea or mucositis or anything– but we’ve seen some patients and it’s the people who have these strange side effects who will post and say, I’m having this. Do you think that there is any correlation between, DPD deficiency and the cardiac kind of symptoms?

Dr. Thomas Holden
That is a good question. To my knowledge, no. So it’s very rare, but yes, it happens, right? This coronary vasospasm, this feeling like I’m having a heart attack and we still have to separately counsel patients on that.

That’s another, I consider it another black box warning for these drugs. And it’s interesting, that side effect does not appear to be dose dependent. So it doesn’t matter if you give them like a baby dose. Doesn’t matter. Like if it happens, it happens. And we can thankfully manage patients through that.

So, I mean, I say that I’m part of an institution where thankfully we have wonderful cardiology colleagues, right? So it happens. We talk with them. They get started on a calcium channel blocker, which is a heart medication for blood pressure, but also reduces stress on the heart. And we can actually then rechallenge patients with the same dose they had before and they can actually get through it without having those terrible, the terrible vasospasm.

So not quite the same thing with DPYD deficiency, where if their enzyme is deficient, they have the severe side effects. I wouldn’t feel comfortable rechallenging them no matter what we do with the same dose. We say we have to go down. It’s too much. But interesting question about the coronary vasospasm. Yeah.

Manju George
Yeah. So you think next time somebody posts, there are people posting saying maybe you should check your DPD deficiency. But based on what you’re saying, you think that that’s a separate side effect. It might not have anything to do or the other thing, right, like dose reduction may not have an impact on that.

Dr. Thomas Holden
Yes, correct. So I’m not necessarily convinced, to my knowledge expertise that it’s a dose thing for the coronary vasospasm. Now, of course, there’s that concern. You worry, you would think it’s common sense. You’re like, well, if I got the full dose of these capecitabine pills or the 5FU I had the coronary vasospasm.

But I would say that, but I, I would say that to your point of like, should you still get a DPYD test, I mean, we’re recommending everyone get tested, so I think you should have it. I don’t think it correlates to my knowledge with the coronary vasospasm I think that’s totally separate.

Manju George
Okay. And then there are some old papers about bolus 5 FU being better than infusional. Do you, have you seen for this coronary vasospasm?

Dr. Thomas Holden
Yeah. I think that in, you know, in my practice we still try to challenge with infusional. I just in general, I think, basically outcomes with cancer are better in patients who are able to undergo regular dosing and infusional dosing. That’s not bolus. So we check for, I would prefer to try to get patients if possible, we try to rechallenge them with actual, infusional over two days with 5 fluorouracil.

Manju George
Okay. Okay. I, I think we have six more minutes. But here’s the other question. So outside us, there is this S-1 , right?

Instead of 5FU. So we have maybe five people in COLONTOWN who they had the severe reaction and their doctors said, no, we are not using 5FU on you at all. But, outside US , there are other drugs that are possible that they, I think the S-1 is one in Japan and maybe in Europe too.

I think that for whatever reason I heard that there’s some story around why no trials or anything was ever done in the US so it’s not available in the US.

Dr. Thomas Holden
Yes, correct. When I was a fellow, I asked my mentors and I asked everyone, you know, ’cause when we go to ask ’em to go to the conferences, you see in Asian countries, they have a lot of abstracts posted about, using

S-1 and I’m not sure if it’s regulatory stuff, just doing studies over here. It never took off here. So we just don’t have it available here. So it’s hard for me to say. It’s hard for me to know. So it’s just something that, it’s hard for me to provide guidance on dosing for S-1 here.

’cause we just don’t, we just don’t use, it’s not FDA approved. It’s not, it’s not been studied here on our patient population. So when we look at S-1, one of the things that when as physicians, when we’re guiding our patients on what to do with treatment, we have to ask, the first question is, when you see a study that looks at a drug, does this apply to the patients that I’m treating? And it’s hard when a drug is only being looked at in one part of the world. So don’t get me wrong. I mean, we look at the data and we’re like, yeah, it, it looks like it’s a great alternative. So they use that in certain cases over 5FU.

So, but how does that apply to my patients? I don’t know. We would need to study the drug over here a little bit more. But it’s just one of those alternatives that’s not like by a mechanism of action, we don’t think it’s vastly superior to anything we have in the US by any means.

It’s just something that, it’s like a sister drug that we just is never caught on here.

Manju George
Okay. Okay. And so far, have you had anybody who’s with complete deficiency and that how do you counsel them?

Yeah, so if someone has complete deficiency, like complete, so if you remember I talked about there’s complete deficiency and then there’s still they have some function in some, but they’re still a very poor metabolizer, so there’s like a 0.5 activity score. I think I handle them both different differently. For people who have complete deficiency, unfortunately we have to find an alternative treatment. It’s very, and that can be very difficult, especially if we’re talking about a patient who is looking to undergo curative intent treatment that requires these drugs.

We try to find the best alternative possible, and that’s a very case by case, discussion for people who have very severe deficiency, but they still have some function of the enzyme. There is a determination where we can reduce the dose of the drug by 75% and very carefully try it. And that’s what we would do because again, these are drugs that are used in curative intent treatment.

If someone has rectal cancer and colon cancer and we’re trying to do a curative treatment, if they have at least some function of the enzyme, we would very safely dose reduce it. And if I give them only 25% of the drug and they do fine. Then I say, let’s increase it a little bit more next time ,of the next round, and we’re gonna try it again to try to get them as much of an effective dose as possible.

Okay. Okay. Okay. Thank you so much. This was very informative. I think we’ve covered every possible scenario, right? Yeah.

Dr. Thomas Holden
Well, I hope so. I hope, yeah, I hope it was, I hope it was helpful. I really, again, thank you so much for giving me the opportunity to come talk to y’all about this.

Manju George
You’re very welcome. And then when the video’s ready, I will send you the link.

Dr. Thomas Holden
Okay. Awesome. Thank you so much.

Manju George
Thanks to everyone attended the session. A video will be posted on COLONTOWN University in the Lecture Hall in about two weeks.

Dr. Thomas Holden
Okay.

Manju George
Thank you very much.

Dr. Thomas Holden
Take care. Bye-bye.