Because topoisomerase (topo) I- and topo II-targeting agents exert their principal effects on the two major classes of enzymes involved in regulating DNA topology in the cell, there has been considerable interest in evaluating combinations of these classes of agents. In preclinical studies of inhibitors of topo I and topo II in combination, drug scheduling and sequencing have been critical determinants of antitumor activity, with a greater magnitude of cytotoxicity generally occurring when treatment with the topo I inhibitor precedes treatment with the topo II-targeting agent. The underlying mechanism that has been proposed to explain this schedule dependency is compensatory up-regulation of topo II and, therefore, enhanced cytotoxicity of topo II inhibitors in cells treated initially with topo I inhibitors. The feasibility of sequentially administering the topo I inhibitor topotecan (TPT) followed by the topo II inhibitor etoposide to patients with advanced solid malignancies was evaluated in this Phase I and translational laboratory study. Fifty patients with solid neoplasms were treated with TPT doses ranging from 0.17 to 1.05 mg/m2/day as a 72-h continuous (i.v.) infusion on days 1-3 followed by etoposide, 75 or 100 mg/m2/day as a 2-h i.v. infusion daily on days 8-10. The combined rate of severe neutropenia and thrombocytopenia was unacceptably high above the TPT (mg/m2/day)/etoposide (mg/m2/day) dose levels of 0.68/100 and 0.68/75 in minimally and heavily pretreated patients, respectively, and these dose levels are recommended for further disease-directed evaluations of TPT/etoposide on this administration schedule. Successive biopsies of accessible tumors were obtained for quantitation of topo I and II levels prior to and immediately after treatment with TPT and prior to and immediately after treatment with etoposide in seven patients. The results of these limited studies in tumors did not fully support the proposed mechanistic rationale favoring the development of this particular sequential TPT/etoposide regimen, because only two of the six patients' tumors in whom topo I was successively measured had either modest or substantial decrements in topo I levels following treatment with TPT, and the principal effect of interest, up-regulation of topo II following treatment with TPT, was clearly documented in the tumors of only one of six subjects in whom successive measurements of topo I were performed. Even in view of the notable objective antitumor activity in three subjects, including a complete response in a patient with colorectal carcinoma and partial responses in one patient each with non-small cell lung and gastric carcinomas, the toxicity and ancillary laboratory results do not provide substantial evidence that sequential treatment with TPT and etoposide might be more advantageous than either TPT or etoposide administered as a single agent.