Two camptothecin-resistant variants of the CEM human leukemia cell line were developed by stepwise selection in camptothecin (CPT) in vitro. The two lines, named CEM/C1 and CEM/C2, were found to be approximately 31- and 970-fold less sensitive to CPT, respectively, than the CEM parental line and variably cross-resistant to the CPT analogs 9-amino-CPT, 10,11-methylenedioxy-CPT, and topotecan. Levels of DNA-protein complex formation resulting from cell exposure to CPT were found to be progressively reduced in the CPT-resistant cells, despite equivalent CPT accumulation in the drug-sensitive and -resistant cells. Nuclear extracts (1.0 M NaCl) prepared from the CEM/C1 and CEM/C2 lines contained 1.5- to 2-fold less DNA topoisomerase I catalytic activity per microgram of protein than did extracts from the drug-sensitive CEM line, in association with altered sensitivity of the enzyme in the CEM/C1 and CEM/C2 extracts to the inhibitory activity of CPT. Only minor differences were noted in the CPT IC50s for the topoisomerase I activity in extracts from the two CPT-resistant cell lines, however, despite the marked differences in cellular sensitivity to CPT. There were notable differences in the level of CPT-induced cleavage of DNA oligonucleotides by topoisomerase I in nuclear extracts from CEM cells compared with the drug-resistant cell extracts, with very little oligonucleotide cleavage induced by enzyme in either drug-resistant cell type, despite the use of very high (100 microM) CPT concentrations. The alterations in topoisomerase I catalytic activity were associated with reduced cellular levels of both immunoreactive topoisomerase I protein (representing 59 +/- 19% [CEM/C1] and 49 +/- 12% [CEM/C2] of that in CEM, respectively) and mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)