Nature and subjectivity of dose-limiting toxicities in contemporary phase 1 trials: comparison of cytotoxic versus non-cytotoxic drugs.

Dose-limiting toxicity (DLT) remains the preferred metric in dose-finding phase 1 trials. Nevertheless, this primary endpoint appears unsuitable for investigating non-cytotoxic drugs. We reviewed 201 recent dose-finding phase 1 trials and compared the DLT defined with cytotoxic (119 trials) and non-cytotoxic drugs (molecular-targeted therapies and immune-stimulant agents; 82 trials). DLT was less frequently identified with non-cytotoxic drugs (52 vs. 89%, p = 0.00005). Myelotoxicity remains the most frequent DLT in studies investigating cytotoxic agents (51%). Myelotoxicity was significantly less frequent in studies investigating non-cytotoxic drugs (14%, p = 0.00003). Skin toxicities (p = 0.038), coagulation perturbation (p = 0.025) and fever (p = 0.025) were the most frequent DLTs in studies investigating non-cytotoxic drugs. Moreover, DLTs identified with non-cytotoxic drugs were less frequently objectively measurable as they were based on biological anomalies (30 vs. 63%, p = 0.0026). Approximately 50% of dose-finding phase 1 trials investigating non-cytotoxic drugs led to DLT and then the maximum tolerated dose being found. However, the nature of these DLTs is different from those described with cytotoxic drugs and less objectively measurable in many cases.