Let's talk about cytotoxic chemotherapy dosing: unravelling adjustments and off‐protocol prescribing

Individualised dosing is important in cancer treatment in real‐world settings and may require departure from trial‐based protocols

Adequate and appropriate dosing of cytotoxic chemotherapy is an important issue, highlighted recently by public concerns about underdosing.1,2,3 Most cytotoxic agents have a steep dose–response curve and narrow therapeutic index. Underdosing may have significant implications for patient outcomes, while overdosing can result in life‐threatening adverse effects.1 Cancer chemotherapy dosing differs from drug prescribing in other areas of medicine. Phase 1 trials in patients with cancer identify safety and dose‐limiting toxicities, and determine the maximum tolerated dose and recommended phase 2 dose. Phase 2 trials focus on efficacy assessment within specific tumour types.4,5 Phase 3 trials, considered the key criteria for approval by drug regulatory agencies, validate the efficacy of the drug compared with a standard of care. A challenge has emerged with new molecularly targeted agents and immuno‐oncology drugs frequently having different toxicity profiles to chemotherapy, which may not be dose or time dependent. Accordingly, the traditional phase 1 and 2 model may not be as adaptable, resulting in evolution of trial design, such as phase 1 trials combining safety and efficacy expansion cohorts.6