PT  - JOURNAL ARTICLE
AU  - W K Eng
AU  - F L McCabe
AU  - K B Tan
AU  - M R Mattern
AU  - G A Hofmann
AU  - R D Woessner
AU  - R P Hertzberg
AU  - R K Johnson
TI  - Development of a stable camptothecin-resistant subline of P388 leukemia with reduced topoisomerase I content.
DP  - 1990 Oct 01
TA  - Molecular Pharmacology
PG  - 471--480
VI  - 38
IP  - 4
4099  - http://molpharm.aspetjournals.org/content/38/4/471.short
4100  - http://molpharm.aspetjournals.org/content/38/4/471.full
SO  - Mol Pharmacol1990 Oct 01; 38
AB  - A camptothecin-resistant subline of P388 leukemia (P388/CPT) was developed by repeated transplantation of P388 cells in mice treated with therapeutic doses of camptothecin. In mice bearing the resistant tumor, a maximally tolerated dose of camptothecin produced no net reduction in tumor cell burden, in contrast to a 5-log cell kill in the parental P388 (P388/S). The IC50 of camptothecin, as determined by colony formation assays of cultured cells, was 8 times greater for the cloned P388/CPT cell line than for P388/S. P388/CPT cells were not cross-resistant to other antineoplastic agents, including topoisomerase II inhibitors. The type I topoisomerases purified from P388/CPT and P388/S cells were identical with respect to molecular weight, specific activity, in vitro camptothecin sensitivity, and DNA cleavage specificity. Camptothecin induced fewer protein-associated DNA single-strand breaks in the resistant cells than in the wild-type P388 cells. Topoisomerase I mRNA, immunoreactivity, and extractable enzymatic activity were 2-4 times lower for P388/CPT cells than for P388/S cells. As resistance to camptothecin developed, topoisomerase I extractable activity decreased, concomitant with an increase in topoisomerase II extractable activity. Furthermore, the appearance of camptothecin resistance was associated with specific rearrangements of the topoisomerase I gene. These results suggest that development of resistance to inhibitors of topoisomerase I can occur by down-regulation of the target enzyme, thus reducing the production of lethal enzyme-mediated DNA damage. The enhanced topoisomerase II activity in these cells suggests that resistance to camptothecin may be overcome by co-treatment with topoisomerase II inhibitors.