Predictive model for plasma concentration-versus-time profiles of investigational anticancer drugs in patients

Plasma pharmacokinetics of the antitumour agents 5,6-dimethylxanthenone-4-acetic acid, xanthenone-4-acetic acid and flavone-8-acetic acid in mice

Although the antitumour agent flavone-8-acetic acid (FAA) exhibits remarkable activity against murine solid tumours, its clinical use has a number of pharmacological drawbacks, including low dose potency and dose-dependent pharmacokinetics. Xanthenone-4-acetic acid (XAA) and its 5,6-dimethyl derivative (5,6-MeXAA) were synthesised during a search for better analogues of FAA. The maximal tolerated doses (MTDs) of 5,6-MeXAA, XAA and FAA in BDF1 mice were 99, 1,090 and 1,300 mumol/kg, respectively. At the MTD, 5,6-MeXAA displayed the following pharmacokinetic properties: maximal plasma concentration, 600 microM; mean residence time, 4.9 h; AUC, 2,400 mumol h 1-1; and volume of steady-state distribution, 0.2 l/kg. All compounds displayed nonlinear elimination kinetics at the MTD, but when the logarithm of the AUC was plotted against that of the delivered dose, the slope of the regression line for 5,6-MeXAA was found to be 1.2 as opposed to 1.4 for XAA and 1.98 for FAA. 5,6-MeXAA thus showed only a slight deviation from dose-independent kinetics. 5,6-MeXAA bound to plasma proteins in a manner similar to that exhibited by FAA, although the plasma concentration of free drug was lower for the former than for the latter. As a consequence, the calculated maximal free drug concentration for 5,6-MeXAA in plasma was 23 times lower than that for FAA.

Concept of maximum tolerated systemic exposure and its application to phase I-II studies of anticancer drugs

The traditional approach to conducting Phase I studies of anticancer drugs is to select a starting dosage for humans based on preclinical data (e.g., mg equivalent of 1/10 LD10 in mice), then empirically escalate dosages in cohorts of patients until the maximum tolerated dosage (MTD) is established. More recently, NCl and EORTC investigators have advocated the use of pharmacokinetic data from preclinical studies to facilitate more rapid dose escalation (e.g., double the dose until the area under the concentration-time curve [AUC] in humans equals the AUC in mice at the LD10). The present paper describes a strategy which builds on the above approach, by extending the application of pharmacokinetic principles to systematically escalate systemic exposure (AUC) instead of dosage in Phase I trials. Human trials are initiated at whatever patient-specific dosage is required to achieve an AUC equal to 1/10 the AUC in mice at the LD10, such that three patients at the first treatment level might receive three different dosages. If no dose-limiting toxicity is observed, the next cohort of patients receives whatever dosage is required to achieve 2 x AUC of the first dosage level, with AUC escalation continuing until the maximum tolerate systemic exposure (MTSE) is reached. By escalating systemic exposure instead of dosage, one adjusts for interpatient pharmacokinetic variability. This strategy will permit more rapid and precise dosage escalations and, more importantly, it should more precisely establish the maximum level of treatment intensity for future Phase II trials.

Human tumor cloning assays: applications in clinical oncology and new antineoplastic agent development

The human tumor cloning assay (HTCA) has been available for preclinical and clinical applications for the last 11 years. This article examines the usefulness of that assay both in the practice of clinical oncology and in the development of new antineoplastic agents. In the area of prediction of response of an individual patient's tumor to a particular antineoplastic agent, in a total of 2274 correlations in a variety of clinical trials, the assay has shown a remarkably good ability to predict whether a patient's tumor would respond to a particular agent (percent true positives 69%; percent true negatives 91%). However, despite this ability to predict these responses, the assay has not yet been accepted for general clinical use, because there have not been rigorous trials to prove the assay will improve patient survival. These rigorous clinical trials are now underway. In the area of drug development, the HTCA has been used to screen for new antineoplastic agents as well as to pinpoint tumor types against which the new agent will be active in phase II clinical trials. As will be seen in this review, the HTCA has been most successful in this area.