Current perspectiveDose-escalation models for combination phase I trials in oncology
Introduction
Designs for phase I trials in general and combination drug phase I trials in particular are facing several challenges with the increasing diversity of classes of agents and mechanisms of action. In addition, these new classes of anticancer agents are often administered continuously rather than intermittently, and they manifest safety profiles that are completely different from those of conventional cytotoxic agents. Determining the best dose for new combination drug treatments will have to be balanced against these aspects. Longer observation periods than the usual 3–4 weeks are required, given that continuously administered drugs may show relevant toxicity only after a prolonged exposure.1 There are currently approximately 850 agents in the development for cancer treatment,2 which could potentially be combined into approximately 400,000 two-drug combinations, not to mention potential combinations involving already marketed agents. Computational modelling of new drug combinations may be a way forward, but as long as this has not been developed, we will have to work with realistic phase I study modelling and planning.
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Rationale
A phase I study of a new combination of drugs is only the first clinical step in the development of that specific combination and should be considered a means, not an end in itself. Before commencing a combination drug phase I trial, a plan for (tumour-type specific) subsequent developmental phase II/III studies must clearly be defined. The choice for tumour type should be based on a scientific rationale, ideally including data in an appropriate preclinical model or on the basis of clinical
It all starts with an hypothesis
A single ideal template to perform a combination drug phase I trial will likely never exist. Every trial has to be designed based on prior knowledge of the preclinical and clinical pharmacology of the individual agents. Given the enormous number of potential combinations, in addition to a proper rationale and further development plan as outlined above, an appropriate hypothesis to guide the protocol design is also crucial. We have identified three potential general hypotheses for such studies.
Defining the MTD in Types 1 and 2 studies
In theory the number of MTDs of two drugs is infinite and does describe a curve, which we can call an envelope of tolerability. Obviously, the full envelope of tolerability of a drug combination will never be described, but defining multiple MTDs as derivatives of the envelope will generate knowledge on the dosing-range of the combination (Fig. 2).
At first glance, the MTD often seems to be determined by the data generated during the trial. However, some choices made with regard to the
MTD/DLT-incidence in perspective of known toxicity profiles
Combination phase I studies should preferably be initiated if knowledge is available on the single agent toxicity profile of the involved agents. Combination phase I trials should aim to model the toxicity as function of dose and PK. While defining the limits of ‘tolerability’ based on an incidence of unacceptable side-effects is relatively straightforward in single agent phase I trials, this turns out to be more difficult for combination phase I studies. And yet defining MTD and DLT is
Avoiding background noise on toxicity
One of the challenges in conducting combination drug phase I trials is to carefully weigh whether or not the frequency of observed toxicity is representative for the novel combination or that, due to chance, the toxicity attributable to just one of the drugs occurs in the trial in a higher than usual incidence.
Two instruments may be helpful by valuing the toxicity data generated more carefully: (1) the 3 + 3 + 3 design and (2) introducing control groups into phase I studies.
Determining feasibility in Type 3 studies
If no PK or PD interaction is anticipated, the feasibility of a combination can be proven in a kind of phase I/II trial. If we assume that drug A is already regarded as standard treatment for a certain tumour type, then patients with this tumour type can be enroled in a trial in which all patients start with a combination of drug A and drug B, both at full dose. Such an approach is feasible in, for example, the non-small lung cancer patient population in which the epidermal growth factor
Conclusion
Dose-finding studies are the first step in the clinical development of new combination treatment strategies. Combination phase I studies are extremely complex. Designing phase I studies is as important as the conduct itself and should be done by dedicated phase I researchers since a standard template cannot be made.
If there is a rationale to develop a specific drug combination, a hypothesis should be generated. This hypothesis, based on the anticipated levels of interaction, will be guiding for
Conflict of interest statement
None declared.
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Present address: Department of Internal Medicine, Sint Franciscus Gasthuis, Rotterdam, The Netherlands.