Dose-limiting toxicity (DLT)

Definition

Dose-limiting toxicity (DLT) is the maximum level of adverse effect (toxicity) that is considered acceptable in the context of a drug's therapeutic benefit. In clinical trials, investigators escalate the dose level in steps (starting at a low dose level and moving to higher levels) while carefully monitoring safety. As dose increases, adverse effects typically increase in frequency and severity. At some dose level, the adverse effects become so severe or frequent that further dose escalation would compromise patient safety unacceptably. This dose level is the dose-limiting toxicity. For example, in a cancer chemotherapy trial, a dose that causes liver damage in 20% of patients might be acceptable because the cancer treatment benefit is substantial; however, a dose that causes fatal liver failure in 30% of patients would be dose-limiting and would not be acceptable. The dose recommended for further development is typically below the DLT, allowing a margin of safety. DLT is relevant for any drug, but it is particularly important for anticancer drugs and other drugs with high intrinsic toxicity. For research peptides intended for non-critical indications, the acceptable level of toxicity is lower than for life-saving cancer drugs, meaning the DLT for a peptide intended for cosmetic purposes might be quite low.

DLT determination is part of phase 1 clinical trials, in which investigators systematically escalate dose levels in small groups of patients (typically cancer patients for oncology drugs, or healthy volunteers for non-oncology drugs) and carefully monitor for adverse effects. Standard designs such as the '3+3' design are used: three patients receive a dose; if none experience DLT, the dose is escalated to the next level and three more patients receive the higher dose. If one or more patients experience DLT, the dose is escalated more slowly or abandoned if already at an unacceptably high level.

Understanding the DLT concept is important for realistic expectations about new drugs. No drug is completely safe; all drugs produce adverse effects at some dose. The relevant question is whether the therapeutic benefit (for an indicated condition) justifies the risks. This risk-benefit calculation differs for different indications: a drug causing serious liver toxicity might be acceptable for a fatal disease but completely unacceptable for a cosmetic indication.