Effects of the combination of camptothecin and doxorubicin or etoposide on rat glioma cells and camptothecin-resistant variants

Abstract

Summary From the rat C6 glioma cell line in culture, we selected camptothecin-resistant variants by growth in the presence of increasing amounts of this drug (C6 CPT10 , C6 CPT50 and C6 CPT100 , growing respectively with 10, 50 and 100 ng ml -1 camptothecin). The degree of resistance to camptothecin ranged between 15-fold (C6 CPT10 ) and 30-fold (C6 CPT50 and C6 CPT100 ). The C6 CPT10 cell line presented a collateral sensitivity to etoposide (3.6-fold), while the C6 CPT50 and C6 CPT100 cell lines were cross-resistant to etoposide (1.8-fold) The resistant lines were characterised by a two-fold reduced content and catalytic activity of topoisomerase I, and C6 CPT50 and C6 CPT100 presented a significant increase in topoisomerase IIα content and catalytic activity and a marked overexpression of P-glycoprotein. We explored the cytotoxicity of combinations of a topoisomerase I inhibitor (camptothecin) and a topoisomerase II inhibitor (doxorubicin or etoposide) at several molar ratios, allowing the evaluation of their synergistic or antagonistic effects on cell survival using the median effect principle. The simultaneous combination of camptothecin and doxorubicin or etoposide was additive or antagonistic in C6 cells, slightly synergistic in the C6 CPT10 line and never more than additive in the C6 CPT50 and C6 CPT100 cell lines. The sequential combination of doxorubicin and camptothecin gave additivity in the order camptothecin → doxorubicin and antagonism in the order doxorubicin → camptothecin. Clinical protocols combining a topoisomerase I and a topoisomerase II inhibitor should be considered with caution because antagonistic effects have been observed with combinations of camptothecin and doxorubicin.

Enhanced inhibitive effects of combination of rofecoxib and octreotide on the growth of human gastric cancer

“…To determine whether the 2 medicines were synergistic, we used the median effect principle. 15,16 Through this analysis, we found synergism between these 2 drugs in the majority of responses, although the combination did not show very drastically enhanced effects when compared with rofecoxib alone in vitro. However, rofecoxib combined with octreotide greatly enhanced the effects of antineoplasm in nude mice in the respect of PCNA or apoptosis.…”

“…The combined effects of rofecoxib and octreotide in terms of synergism, summation, or antagonism were quantitatively analyzed by the median effect plot derived by Chou and Talalay 15,16 as follows: fa/fu ϭ (D/Dm) m , where fa indicates response, fu unresponse, D the dosage, Dm the dosage of half inhibition, m the slope and fu ϭ 1 Ϫ fa. Interaction of the 2 drugs is quantitatively determined by the combination index (CI), which is calculated as follows: 2 , where D X is the dose that is required to produce X% inhibition of 3 H-TdR incorporation.…”

“…Then, each group was exposed to 37 kBq/ml of [methyi-3 H]-thymidine (Amersham Pharmacia Biotec, Piscataway, NJ) in the same medium for 2 hr. The radioactivity was measured by liquid scintillation counter (Becman LS1801).The combined effects of rofecoxib and octreotide in terms of synergism, summation, or antagonism were quantitatively analyzed by the median effect plot derived by Chou and Talalay 15,16 as follows: fa/fu ϭ (D/Dm) m , where fa indicates response, fu unresponse, D the dosage, Dm the dosage of half inhibition, m the slope and fu ϭ 1 Ϫ fa. Interaction of the 2 drugs is quantitatively…”

Exposure to low concentrations of etoposide reduces the apoptotic capability of leukaemic cell lines

“…This family consists of many members, however, the specific efflux of etoposide is mediated primarily by p-glycoprotein (Pgp), 23,24 and as a consequence, its capacity to efflux the Pgp-specific polymethine dye DIOC2(3) was examined in our cell lines and shown to be identical in all respective cells. The substitution of DIOC2(3) with carboxyfluorescein diacetate, a specific substrate for the multidrug resistance protein, also showed the rate of efflux by this pump to be similarly unchanged in the sublines (data not shown).…”

Improving combination cancer therapy: the CombiPlex ® development platform

“…The implication of this is that the relative concentration of each agent exposed to the cancer cell (molar drug ratio) is important. Drug combinations may act synergistically, additively or antagonistically, depending on the molar ratios of the individual agents comprising the combination [7,8]. Additive drug ratios result in activity that is equal to the sum of the effects contributed by each drug [6].…”