Purpose: One tenth of the lethal dose to 10% of mice is one of the conventional parameters used to derive a safe starting dose in phase I trials of cytotoxic agents. There is no consensus on which preclinical models and parameters should define the starting dose for molecularly targeted agents.
Patients and methods: Reports of 81 first-in-human phase I trials evaluating 60 different molecularly targeted agents administered as monotherapy were reviewed. The maximum-tolerated dose (MTD) was defined as the highest safe dose administered to patients, whereas the maximum-administered dose (MAD) was recorded if the MTD was not reached.
Results: Fifty-seven of the 81 trials specified the animal model used to determine the starting dose, with 29 (51%) of 57 based on rodent data and 28 (49%) of 57 based on non-rodent data. A wide range of toxicologic parameters was used to select the starting dose. The starting dose exceeded the human MTD in three (3.7%) of 81 trials, and in all three trials, nonhematologic toxicity was dose limiting. The median number of dose levels to reach MTD or MAD from starting dose was five (range, one to 14 dose levels), and the median ratio of MTD or MAD to starting dose was 12 (range, < 1 to 300). Hypothetical doubling of the starting dose appeared to be safe, whereas tripling of the starting dose was unsafe.
Conclusion: The derivation of starting dose for first-in-human phase I trials of molecularly targeted agents in patients with cancer is safe but is based on diverse practices using a variety of preclinical toxicologic parameters.