Shrinking oncology phase I clinical trials: If the drug is a breakthrough, why would you ever want to utilize the maximally tolerated dose?

Historically, oncology drug development has evolved on what may seem to be a different planet, at least relative to mainstream clinical pharmacology.

Whereas traditional drug development utilizes standard tools of randomization, pharmacokinetic data, and blood-based biomarkers to optimize dosing, oncology has primarily utilized safety data (i.e., maximally tolerated dose) to guide dose selection.

In this context, I and others have advocated for the use of expansion cohorts to try to precisely define the maximally tolerated dose, although such estimates are often unreliable due to small cohort sizes (1-4).

While this historical paradigm may have been appropriate for cytotoxic chemotherapy agents, it is certainly not appropriate for precisely targeted enzyme inhibitors or monoclonal antibodies, comprising much of modern oncology drug development.

The lack of proper dose selection in oncology has led to a number of post-marketing requirements to study lower doses, with many of these studies leading to label changes (5). It has also created an opportunity to reduce prescribing costs using lower dosages or less frequent dosing off-label (6-10).

While this historical paradigm may have been appropriate for cytotoxic chemotherapy agents, it is certainly not appropriate for precisely targeted enzyme inhibitors or monoclonal antibodies, comprising much of modern oncology drug development.

The few studies published to date represent only a small fraction of the opportunity to reduce prescribing costs through off-label dosing, an opportunity created by the industry’s lack of rigor in dose selection(11), and leading to the formation of the Value in Cancer Care Consortium.

A recently published FDA draft Guidance (12) now seeks to advise companies on approaches to optimize phase I clinical trial designs with particular attention to the increasing and widespread use of expansion cohorts. While concerns regarding the size of some phase I trials have been longstanding (13), the recent draft Guidance may have been motivated by Merck’s phase I study of pembrolizumab, comprised of 1,235 patients (14).

The draft Guidance augments the information in the Code of Federal Regulations (21CFR312.21) that explicitly articulates the aims and appropriate sizes of phase I, II, and III trials. Notably, phase I studies are stated to be “generally in the range of 20 to 80”, given the goal of obtaining “sufficient information about the drug’s pharmacokinetics and pharmacological effects… to permit the design of well-controlled, scientifically valid, phase II studies.” In this context, there is no suggestion that phase I studies can or should identify the optimal dose, and in fact this is generally (i.e., outside oncology) ascertained in the context of randomized dose-ranging phase IIb studies (15).

The draft Guidance appears to suggest that expansion cohorts are overused, and should only be considered in the context of a breakthrough drug. It logically follows that in the absence of clinical activity suggesting breakthrough activity, that the sponsor should follow a more traditional approach of completing and fully analyzing a phase I trial before beginning phase II studies.

The draft Guidance also articulates reasons not to utilize expansion cohorts even if the drug demonstrates potential breakthrough activity, such as steep toxicity indices or high interindividual or intraindividual variability. Given the need to exclude high variability, this suggests that expansion cohorts should not be initiated until there are sufficient pharmacokinetic data available, including data regarding intraindividual variability, a particular problem for many small molecule kinase inhibitors (16).

Furthermore, the draft Guidance explains that should a sponsor wish to conduct one or more expansion cohorts, that each of these have a clear scientific rationale and statistical plan, and that FDA input should be sought early and often. Notably, the draft Guidance also raises concerns regarding the adequacy of safety monitoring for expansion cohorts, and mandates the incorporation for an independent safety assessment or data monitoring committee for all first-in-human multiple expansion cohort protocols.

Thus, my interpretation of the FDA’s position is that protocols with expansion cohorts should be the exception and not the rule, that FDA review of such protocols is required, and that if there truly is a compelling reason to study multiple expansion cohorts, that there will be many additional study requirements in the interest of both protecting patients and generating scientific knowledge.

If my interpretation is correct, I am enthusiastically supportive of the FDA’s recommendations. Phase I trials have gone from being proper clinical pharmacology studies conducted by experienced investigators at single institutions to massive multi-center studies run by contract research organizations, and often overseen by medical monitors with limited experience in drug development. The site investigator’s responsibilities are limited to enrolling patients and completing case report forms. Manuscripts are drafted by a scientific writer employed or contracted by the sponsor, and peer reviewers are generally more focused on efficacy results than the clinical pharmacology of the agent under study.

My hope is that the draft Guidance will be finalized soon with minimal or no revision, and that sponsors will be forced to revisit the historical principles of clinical pharmacology that are widely utilized in all therapeutic areas other than oncology. And to the sponsors out there: if your drug is so good, why would you ever want to utilize the maximally tolerated dose?

REFERENCES

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12. FDA. Expansion Cohorts: Use in first-in-human clinical trials to expedite development of oncology drugs and biologics guidance for industry DRAFT GUIDANCE 2018. Available from: https://www.fda.gov/ucm/groups/fdagov-public/@fdagov-drugs-gen/documents/document/ucm616325.pdf.

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