“Weight of Evidence vs Carcinogenicity Testing (Part 2)” Featuring Bruce Pearce

Pharmacology and toxicology expert, Bruce Pearce, continues sharing his insight about the new FDA S1B guidance R1 addendum. These changes may allow you to forgo carcinogenicity testing in favor of a weight of evidence argument. On this episode of Insight at Biologics, we explore the many factors involved, including the 2 year rat and rasH2 mouse studies, pharmacologic mechanisms, toxicology studies, and more.

Transcript

James: In our previous episode, we were talking with Bruce Pearce about the new ICH guidance, which allows for using a weight of evidence argument in place of a two year rant carcinogenicity study. But how do you make a weight of evidence argument? Now, Bruce will talk about that in just a moment, and this is insight at Biologics.

First, a bit of housekeeping. If you’d like to know the breadth of Bruce’s credentials on this subject, you can listen to him tell you all of that himself in the first part of this interview, the previous episode, or you can email us at insight@biologicsconsulting.com. Now let’s get back to it, Bruce. What are the specific factors to consider that are recommended for incorporation into this integrated WOE. (weight of evidence)  argument/document?

Bruce: Thank you for the question, James. The weight of evidence integrated approach, as I said in the previous podcast, is based on a comprehensive assessment of the totality of data relevant to the carcinogenic potential available from public sources, and from the results of relevant drug development studies with a sponsor’s product.

But what’s important are, what are the factors as you’ve targeted to consider in this evaluation? And these are in fact, the elements that should be addressed as a part of putting together a weight of evidence argument. And there are five of them that I will discuss here briefly. The first is data that informs carcinogen genetic potential based on the drug target, the primary pharmacologic mechanism of action of the parent compound.

And I’ll mention here for the first time, the major human metabolites. So, If you have a major metabolite identified in the metabolism of your parent compound, this will need to be assessed as well. Included in this assessment related to the primary drug target as the distribution of the target, for example, in rats and humans, is the target present in the rats that your pharmacologic agent is acting on is the pharmacologic activity and the potency of the parent compound or the major metabolites, if appropriate in these species.

Information from genetically engineered models, human genetic association studies and cancer gene databases – this is the idea of the use of gene expression biomarkers, or genomic biomarkers to help to understand tumorigenic potential is something that’s new that’s being examined. In fact, last year in 2022 in Toxicological Sciences, a paper was published, I think it was in April, that discusses this collaboration to use the genomic tools to better predict carcinogenic potential.

And these kinds of data can be used to substantially enhance the weight of evidence arguments. The other element here in terms of the target, or the particular class of drug that it belongs to, is the information on other drugs in the class, which can be very, very important and adds substantially to the weight of evidence arguments one way or the other in terms of carcinogenicity. 

The second element that I want to factor or consider, are the results from secondary pharmacology screens that are typically done early on in the assessment of a candidate that inform in terms of drug selectivity for targets and off-target, especially, for example, if the drug interacts with a nuclear receptor or biotechnology, our data from repeat dose toxicology studies. In fact, there’s an emphasis on, as I mentioned before, the six month rat study to support the two year study, including information about plasma exposure, margins, parent drug, and again, major metabolites. 

An important element is the histopathology findings and data, because in a number of cases you can see histopathological changes that suggest effects that have carcinogenic potential when you don’t have effects on other markers, say hormone levels or something of the kind. The third element is evidence of hormonal perturbation, including knowledge of the drug target, compensatory endocrine responses. You may see in this regard weights and gross microscopic changes in endocrine and reproductive organs from repeat dose toxicology studies.

Relevant results from reproductive toxicology studies where hormones play a preeminent role in the changes that are occurring. And findings from rat studies suggesting hormonal predation may include, as I have alluded to, microscopic changes. These include atrophy, hypertrophy, hyperplasia, biologically significant endocrine and reproductive organ weight changes. Factors that can’t be addressed in terms of changes that may have been contributed, associated with stress as a part of the process of the testing. 

Such findings are suggestive of potential carcinogen genetic risk, and unless investigated for human relevance and demonstrated otherwise, are treated that these genomic models that are being assessed now in this collaboration may prove very important in terms of addressing the human relevance, for example. 

And the fourth point to consider is genotoxicity testing. There’s a series of studies that are done that complement the full package for genotoxicity testing and any equivocal genotoxicity results that can’t be resolved would further implicate the potential for carcinogenic risk. And finally, modulation that is broad immunosuppression may provide significant concern for human risk. 

James: All right, so Bruce, in your view, examination of these factors that you’ve just delineated, would that be sufficient to support a weight of evidence argument? 

Bruce: Yes, it very well could. Evaluating these factors may be sufficient to conclude whether or not a two year rat study would add value to the assessment of human carcinogenic risk.

However, the weight that’s attributed to each of these factors may depend on the nature of the molecule, so it’s not so straightforward, however, one or more weight of evidence factors may be inconclusive or indicate a concern for carcinogenicity. The sponsor can apply investigative approaches that could address the uncertainty or inform human relevance of identified risk.

I just wanna pause here for a second and emphasize here again, is a situation where you need time to perform these other investigative approaches to be able to address this adequately. This can’t be an afterthought and try to do this at the end of your program. 

James: Okay, so why, why don’t you explain a little bit more about the other investigative approaches that are appropriate?

Bruce: Yeah, some of the other approaches may involve other investigative studies as I’ve mentioned, or analysis of specimens collected from prior studies using special histochemical stains, for example, looking for molecular biomarkers, serum hormone levels, immune cell function, in vitro, in vivo test systems, and data from emerging technology like the genomic methods that I’ve described. 

All of these really require some forethought, some looking at these in retrospect, very difficult to have samples that you can appropriately stain, measure the biomarkers and serum hormone levels if you haven’t ensured that this was done at the appropriate timing when these studies were performed.

And also, generated to inform human mechanistic relevance at therapeutic doses and exposure. So again, it should be self-evident from this discussion that to be able to pursue these approaches, you need time and you need foresight early on to ensure that you’ll not miss the opportunity to collect these data, both preclinical and clinical.

James: All right, now if I recall, there’s a difference in the availability of information based on the type of substance being explored. For instance, a drug that is a member of an established class versus a novel first in class drug that alters how much information is available. 

Bruce: Yes, that’s exactly right. And, if you were studying a molecule that has other members of the class, you can leverage that data to really add substantially to your weight of evidence argument, particularly if carcinogenicity has been seen among the other members. That might really be one of the primary elements supporting your conclusion that it’s highly likely that it’s carcinogen genetic. 

But, nevertheless, novel drug targets, which you refer to, can be included in this weight of evidence argument. The problem is that the bar is higher. Further evidence that there’s no cause of concern regarding the target biology is needed when there’s a lack of other members of this class, or a lack of precedent, if you will, for the novel drug that you’re studying. And this has been addressed in part, in this addendum, there is a case study that’s described in the appendix to the addendum, I guess an example of a novel target where a two year study was not considered to add value. 

And the evidence compensated for this lack of precedent. In this example, the cause for carcinogenic concern was not identified regarding the drug target biology, the compound selectivity. There’s no proliferative changes in any organs or tissues, and were observed at high multiple exposures in the six month rat study. So, the results of this were very clean. This is a particular idyllic example, if you will, but it’s actually drawn from the data from those 45 molecules that were studied to assess the ability of the use of weight of evidence in lieu of two-year study. 

When the weight of evidence assessments supports a conclusion that the conduct of a two-year rat study does not add value to the assessment of human risk, the ICH guidance indicates that the sponsor should seek consultation with the appropriate regulatory agency in accordance with the established procedures for that particular area, and they further state that, if the sponsor decides to conduct a two year rat study in accordance with the S 1B guidance, there’s no obligation to seek further consultation with the drug regulatory agency. 

But, my opinion is with respect to US FDA, it is submission of the proposed protocols for both the rasH2 mouse study, and the two year rat carcinogenicity study to the appropriate division for review by the Carcinogenicity Assessment Committee and executive is advisable before initiating these studies and committing two plus million dollars to performing this study. 

You don’t wanna be in the position that after you’ve spent all of this time and invested this kind of money, that the FDA reviewers inform you that your study’s not acceptable. So I generally argue that it’s always smarter to talk with the division and give feedback from the reviewers before you take any major steps in drug testing. 

James: Now, I like examples, Bruce. And so if you would, would you give us some examples of how these principles are applied in cases that come to each of the conclusions? 

Bruce: Yeah, as I mentioned before, there are case studies that are contained in the appendix to this addendum, and I talked to you about the case where it was a novel drug and a novel target, and the conclusion was that an additional carcinogenicity study was not required, and that was case study number four. 

Case studies one and two are examples of pharmaceuticals where the weight of evidence argument indicated that it would not add value to the assessment of human risk, and case study number three described data from weight of evidence assessment of the carcinogenic potential, where the conclusion was uncertain there in that situation, where a two year carcinogenicity study was recommended. 

I think it’s important to understand that, while these examples are taken from real world data, from the studies that were evaluated by this expert working group, the fact of the matter is, it can’t possibly take into consideration all of the circumstances that would occur with unique drugs. So this provides guidance but doesn’t necessarily cover all the bases. 

James: All right. Now, based on what you said, let’s say I believe that conducting the two year rat study is unlikely to contribute to further understanding of the carcinogenic risk. What factors should I focus attention on then, when preparing the weight of evidence document to support that?

Bruce: Yeah, good question. One of the things that was done in putting this addendum together is that the weight of evidence attributes of a compound that are more likely to support the conclusion, the results of a two year rat study would not contribute value to human carcinogenicity risk assessment.

And I’ll go through these and I think it helps to clarify the points to consider that I talked about before. In the case of the target biology, when it is well characterized, not associated with cellular pathways known to be involved with human cancer development. These are situations that occurred in the case of those 45 molecules that were assessed where there were non mammalian targets and or the carcinogenicity data were available with other drugs at the same pharmacologic class.

Two, there were no identified concerns from secondary pharmacology points to consider. That did not inform any off-target potential for their particular pharmaceutical. Three, the results of chronic toxicity studies indicated that there were no hyperplastic, hypertrophic, atypical cellular alterations or degenerative changes without adequate explanation of the pathogenesis or human relevance indicative of no or on or off target potential of carcinogenic concern. 

Again, this emphasizes the fact that even though you may have these observations, if you can adequately explain them mechanistically and address whether or not there’s any human relevance, it’s important to the contribution to your argument that an additional two year study is not required.

Number three, no perturbation of endocrine or reproductive organs was observed or endocrine findings that were not adequately explained with respect to the potential for human relevance. The overall excess assessment of genotoxic potential was concluded to be negative. And finally, there was no evidence of immune modulation or immunotoxicity based on the repeat dose toxicology studies.

So this review of these factors to consider give you an indication of the kinds of findings that would help you to make an argument or the nature of the findings that would help you make an argument that a two year rat study would not be required. 

James: All right, now, the six month rasH2 mouse study. Can you use that to support a weight of evidence argument or even in place of the weight of evidence document and the two year study. 

Bruce: Yeah. Thank you, James. That’s a very good question. The results of the rasH2 mouse study are not required to be included in the weight of evidence document, but if the results of the rasH2 transgenic mouse study are available, they should be a part of the weight of evidence document.

That just in of itself, indicates that the data from the rasH2 transgenic mouse study is not enough to satisfy the requirement. You need to provide the weight of evidence carcinogenicity assessment document that adequately supports your conclusion of whether or not a study is required.

The other thing I wanted to point out, and this is something that we haven’t touched on yet, is that the rasH2 or other mouse study may not be appropriate, such as when the weight of evidence assessment strongly argues that there’s no carcinogenic risk to humans, even before you do a study like that, or when the data indicate that only sub-therapeutic and pharmacologically inactive levels of drug compared to the human exposures can be achieved in the mouse model.

And finally, when weight of evidence assessment indicates that a compound is likely to be carcinogenic in humans. So aside from these elements that we’ve talked about here, there are other circumstances that could contribute to whether or not performing one of these studies is going to add any value in terms of understanding risk in humans.

James: All right. Now, we have talked about a lot, so much that we filled up two episodes, but how would you boil it all down for them? 

Bruce: Well, there are, there are a couple of things that I want to touch on before we end this rather than going through and reviewing all the elements that we’ve already talked about.

James: All right. Go ahead. 

Bruce: I want to touch on first, with respect to having a clear understanding of the testing that will be required for a particular product. I encourage companies once they’ve determined based on the nature of their product, of course, the clinical indication, duration of treatment, that carcinogenicity assessment is required, and have at least formulated a sound strategy to satisfy this requirement to consult with the division handling their particular product. 

Again, I’m encouraging early interaction with FDA, getting feedback from them in this process and to be thinking about this early on. I also hope that the major takeaway from this review is that the development of a strategy for addressing carcinogenicity assessment needs to begin very early for a number of reasons.

First, it’s very likely that the data from studies that will be performed during the development of the clinical testing, which examined the pharmacology and toxicology of the product, would or could generate data important to preparing an adequate weight of evidence document. You want to be asking in real time if there is any data that could be obtained in the course of performing these studies that are potentially important to the weight of evidence documents such as tissue staining, looking at these biomarkers, as I touched on previously. This takes forethought. 

Second, you need early on to assess the metabolites of the parent drug or its excipients in the drug formulation to assess whether it requires carcinogenicity testing as well. Each of these may need to be assessed separately, including a submission of weight of evidence documents for each one of these. The metabolites that are considered major metabolites, and maybe even one or more excipients in your formulation. 

So, this could add a very significant increase in the level of complexity in the overall program. It needs to be dealt with early on to put together a plan where you can utilize this weight of evidence argument in lieu of doing these two plus million dollar rat studies. You may need to perform specialized studies or analysis of specimens, which I touched on before, which almost certainly will require additional time and need to be appropriately designed, performed, and analyzed in the results of analysis in a way that you know will be adequate for the FDA. The ladder may require additional type C meetings or meetings with FDA.

Third, it’s really too late to wait until the 11th hour for this to be an afterthought to address these requirements and most unfortunate to find yourself in a situation that at the end of your development, a testing program, that you need to go back and repeat a six month toxicology studies or pursue mechanistic studies to support your weight of evidence arguments in a situation where you’re convinced that a two year study is probably not needed, but you have to have the data to support your position. 

These studies will require considerable thought in planning as well as the time to establish a consensus with the FDA, including the Carcinogenicity Assessment Committee, the CAC. Sponsors should plan to establish a dialogue and agreement with FDA reviewing what is necessary to satisfy this requirement and the timing of submission of study reports, certainly prior to an NDA or a BLA submission, and preferably by the time of the pre BLA and pre NDA meetings.

It may be possible also to negotiate post-marketing commitment for completing and performing the required studies. However, waiting to address this or other issues until the 11th hour will almost certainly be associated with delays to market and loss in significant revenue. 

So Biologics would like to encourage sponsors to be proactive in addressing these requirements, particularly their strategy for satisfying the requirement for carcinogenicity testing early on in their development process, and my colleagues and I are here to help shepherd sponsors through this process so that they avoid the pitfalls discussed above and substantially reduce risk. 

James: All righty. Thank you, Bruce. I appreciate you coming and talking about that. And as you said, we are available to help you with this and anything else in the world of biologic development and drug development.

So if you’d like to talk to us, just send us an email at insight@biologicsconsulting.com. That’s Insight, @ Biologics Consulting, all one word, .com. Thank you again, Bruce. We’d also love it if you’d like, subscribe to, and rate and review our show. 

The executive producer of Insight at Biologics is Kris Kraihanzel. This episode was produced and edited by James C. Taylor and the technical supervisor is Jeff Weiss. The Insight at Biologics theme is by Tom Rory Parsons. I’m James C. Taylor. Thank you for joining us and please come back for more insight at biologics.