The effects of verapamil and a tiapamil analogue, DMDP, on adriamycin-induced cytotoxicity in P388 adriamycin-resistant and -sensitive leukemia in vitro and in vivo

Pregnane Glycosides

Pregnane glycosides constitute a class of compounds widely distributed in the plant kingdom. Many of them have shown either anticarcinogenic or cancer inhibitory properties, besides other useful biological activities. New chromatographic techniques and advances in spectroscopic and spectrometric methods have accelerated the purification and structure determination of novel glycosides of this series. A compilation of the pregnane glycosides isolated from 1995 until the middle of 2005, along with their physical data, structures and occurrence are presented in this review, which also summarizes, with suitable examples, recent developments in isolation and purification techniques, and structural elucidation using modern spectrometric methods like ESIMS and tandem mass spectrometry, and 2D NMR spectroscopic strategies. The reported anticancer and other biological activities of pregnane glycosides are also discussed.

Drug interaction studies between paclitaxel (Taxol) and OC144-093--a new modulator of MDR in cancer chemotherapy

The MDR modulator, OC144-093, is a potential candidate for use in cancer therapy and exhibits potent biological activity in vitro and in vivo when combined with anticancer agents such as paclitaxel. Its inhibitory interaction with P-glycoprotein (Pgp), the mdr1 gene product and a mechanistic participant in multidrug resistance, underlies its activity as a modulator of MDR. Having previously shown that OC144-093 is not a substrate for CYP3A we first examined the effects of OC144-093 on paclitaxel metabolism in vitro. Using human liver microsomes, we have demonstrated that OC144-093 inhibited the CYP3A mediated metabolism of paclitaxel at high concentrations only (Ki = 39.8 +/- 5.1 microM, n=3). Pharmacokinetic results also show that an oral dose of OC144-093, co-administered with paclitaxel caused negligible disturbance of the pharmacokinetic profile for paclitaxel when injected intravenously. In contrast, AUC values were elevated approximately 1.5-fold in all groups treated orally with paclitaxel and OC144-093. Cmax was enhanced approximately 2-fold in the co-dosed group. These characteristics are consistent with Pgp blockade in the gut enhancing oral bioavailability. Elimination properties of paclitaxel were affected only upon multiple dosing of OC 144-093. These results warrant the further clinical assessment of OC144-093 as an MDR reversing agent.

Distribution and activity of doxorubicin combined with SDZ PSC 833 in mice with P388 and P388/DOX leukaemia

SDZ PSC 833 (PSC 833) is a non-immunosuppressive analogue of cyclosporin A and is a potent modifier of P-glycoprotein (P-gp)-mediated multidrug resistance. The present study was undertaken to evaluate whether doxorubicin (DOX) pharmacokinetic and anti-tumour activity on P388- and P388/DOX-resistant leukaemia was modified by PSC 833 pretreatment. P388- or P388/DOX-bearing mice were given PSC 833 intraperitoneally 30 min before an intravenous injection of DOX. The levels of DOX were determined by a high-performance liquid chromatography method in leukaemic cells and in normal tissues (heart, lung, liver, small intestine, kidney and spleen). In all tissues, DOX concentrations were significantly increased in mice pretreated with PSC 833. The difference was greatest in P-gp-overexpressing P388/DOX cells, the DOX area under the curve being approximately seven times greater after PSC 833 and DOX than after DOX alone. In P388 cells the difference was approximately 2.5 times, as in the majority of normal tissues. As expected DOX levels in P388 cells were higher than in P388/DOX cells in mice treated with DOX alone, whereas after PSC 833 and DOX the levels of DOX were similar in the two leukaemic lines. In spite of this PSC 833 was unable to reverse the resistance to DOX of P388/DOX leukaemia in vivo, suggesting that mechanisms other than P-gp expression are responsible for resistance.

The dithiane Ro 44-5912 enhances vinblastine sensitivity of drug resistant and parental KB lines in vivo

The multidrug resistance modifying activity of a dithiane analogue of tiapamil, Ro 44-5912, was examined in vivo. Results of acute toxicity studies in mice indicated that lethal toxicity occurred with doses greater than 1 mmol/kg of body weight. In a preliminary pharmacokinetic investigation, Ro 44-5912 appeared to have a longer half-life in mice than did its (R) enantiomer Ro 44-5911 (3.15 +/- 0.02 h versus 2.15 +/- 0.14 h) as measured by total radiolabel in plasma. In non-tumour bearing mice, Ro 44-5912 enhanced the toxicity of vinblastine in a manner that was dependent on the dose of both drugs. Vinblastine did not have a significant effect on tumour growth when given to nude mice bearing the parental cell line KB-3-1 at a dose of 1.5 mg/kg once per week for 3 weeks. Combination treatment with Ro 44-5912 markedly enhanced the antitumour activity of vinblastine. Similar results were seen when KB-3-1 tumours were treated with the combination of vinblastine plus cyclosporin A. Another tiapamil analogue, Ro 11-2933, had no enhancing activity with this tumour when used at an equitoxic combination dose. Ro 44-5912 also significantly enhanced vinblastine activity with P-glycoprotein-expressing KB-8-5 tumours. In three independent experiments, Ro 44-5912 enhanced the growth inhibiting activity of vinblastine by a mean of approximately 40%. Neither Ro-11-2933 nor cyclosporin A, at the maximal tolerated doses in combination with vinblastine, led to significant inhibition of KB-8-5 tumour growth compared to treatment with the two vehicles alone. These results show that Ro 44-5912 is an active modulator of drug resistance in vivo.

Multidrug resistance in the laboratory and clinic

Multidrug resistance represents a major obstacle in the successful therapy of neoplastic diseases. Studies have demonstrated that this form of drug resistance occurs in cultured tumor cell lines as well as in human cancers. P-glycoprotein appears to play an important role in such cells by acting as an energy-dependent efflux pump to remove various natural-product drugs from the cell before they have a chance to exert their cytotoxic effects. Using the tools of molecular biology, studies are beginning to reveal the true incidence of multidrug resistance, as mediated by the MDR1 gene, in the clinical setting. It has been demonstrated, at least in the laboratory, that resistance mediated by P-glycoprotein may be modulated by a wide variety of compounds, including verapamil and cyclosporine A. These are compounds which, by themselves, generally have little or no effect on the tumor cells, but when used in conjunction with antineoplastic agents act to decrease, and in some instances eliminate, drug resistance. The mechanism(s) by which these agents act to reverse resistance is not fully understood. Clinical trials to modulate P-glycoprotein activity are now under way to determine whether such strategies will be feasible. The detection of the P-glycoprotein in patient samples is very important in the design of these studies, as it appears that drug-resistant cells lacking P-glycoprotein will be unaffected by agents such as verapamil. Clinical studies are needed in which patients are stratified into chemotherapy protocols based on levels of MDR1 mRNA or P-glycoprotein expression in the primary tumors. Several research areas have been identified that are important to the transfer of the discovery of the MDR1 gene and its protein product from the research laboratory to the clinical environment. There is an immediate need for comprehensive information on the prevalence and levels of expression of the human MDR genes and their protein products in human organs and tissues. Data are needed on P-glycoprotein levels in specific subpopulations (e.g., according to age, sex, race, and diet), and the study of the heterogeneity and variability of expression of P-glycoprotein in normal human tissues should be given high priority. Since early studies have indicated some successes in identifying patients with classic multidrug resistance who might be responsive to chemosensitization, it can be anticipated that clinical research will accelerate in this area. The next wave of clinical studies will provide clinical investigators with opportunities to develop and evaluate P-glycoprotein tests and correlate test results with clinical outcomes.

Differential sensitivity of multi-drug-resistant and -sensitive cells to resistance-modifying agents and the relation with reversal of anthracycline resistance

Calcium channel blockers, calmodulin inhibitors, and some other classes of non-related compounds reverse multi-drug resistance. In the present study, we found that several resistance modifiers are more toxic for MDR cells than for the corresponding sensitive parent cells, whereas others show the opposite effect. Several calcium channel blockers including bepridil, diltiazem, nifedipine and verapamil, as well as the calmodulin inhibitor trifluoperazine, were more toxic for several MDR cell lines than for the parent cell lines. In contrast, cross-resistance for cyclosporin A and the verapamil analogue Ro II-2933/001 was observed in the MDR/sensitive cell couple CHRC5/AUXB1, probably due to a concentration-dependent stimulation of cell growth in the range of 0-4 microM cyclosporin A and of 1-4 microM Ro II-2933/001. In partially revertant CHRC5 cells, growth inhibition by Ro II-2933/001 at concentrations below I microM, as seen in CHRC5 cells, changed into growth stimulation, and the collateral sensitivity to verapamil and bepridil disappeared almost completely. In the MDR cells CHRC5, 2780AD and DC3F/DMXX, cross-resistance to another calcium channel blocking agent, Ro II-1781/001 (tiapamil), was observed as well. This compound showed exceptional behavior: it induced marked potentiation of Dx cytotoxicity as well as stimulation of Dx accumulation in AUXB1 cells, even at low tiapamil concentrations, but not in the CHRC5 cells, even at high concentrations. It is concluded that resistance modifiers can selectively influence growth of MDR cells via more than one process, and resulting in either strong growth inhibition in MDR cells relative to the effect on sensitive cells or in growth stimulation.

A phase II study of intensive-dose epirubicin/verapamil as induction therapy followed by intensive-dose ifosfamide for advanced breast cancer

Preliminary data of an ongoing phase II-study in metastatic breast cancer patients are presented. Patients with metastatic breast cancer entered the study after hormone therapy had failed; prior treatment with cyclophosphamide, methotrexate and 5-fluorouracil (CMF) was also allowed. The patients were treated with three cycles of 40 mg/m2 epirubicin i.v. on days 1-3 and 4 x 120 mg oral verapamil on days 0-3, given every 3-4 weeks. After three chemotherapy courses, ifosfamide was given as a short infusion of 3 g/m2 on days 1-3. Mesna (20% of the total ifosfamide dose) was given 0, 4 and 8 h after ifosfamide administration. Response was evaluated after three cycles of epirubicin/verapamil and after the last (third) cycle of ifosfamide. The side effects of this treatment were tolerable. The epirubicin/verapamil combination was no more toxic than epirubicin alone. Despite the high dose of verapamil, systolic blood pressure remained above 80 mm Hg, and patients never had a period of strict bed rest. Alopecia was almost complete after induction therapy with epirubicin/verapamil, and nausea and vomiting were absent or mild during epirubicin/verapamil chemotherapy and were easily controlled by antiemetics during ifosfamide treatment. Stomatitis and mucositis, the main toxic effects, could be ameliorated by intensive mouth-washing procedures. The haematological toxicity was greater after epirubicin/verapamil treatment than after ifosfamide therapy, but neither bleeding nor infections due to thrombocytopenia or leucopenia were observed.

Modulation by verapamil of vincristine pharmacokinetics and toxicity in mice bearing human tumor xenografts

The effect of the calcium channel blocker verapamil (VRP) on the accumulation and retention of vincristine (VCR) has been examined in mice bearing xenografts of human rhabdomyosarcomas. The tumors were Rh18, moderately sensitive to VCR, and its subline, Rh18/VCR3, selected in vivo for primary resistance to VCR. Administration of VRP by i.p. bolus at dose levels above 75 mg/kg was limited by acute lethality. At this dose, the maximal concentration in plasma was 24 microM, with rapid elimination such that plasma concentrations reported to modulate resistance in vitro (approximately 5-10 microM) were maintained for less than 60 min. To sustain a 10 microM plasma concentration, mice were infused with VRP at 6.25 mg/kg/hr (150 mg/kg/day) for up to 7 days using osmotic pumps implanted in the peritoneal cavity. Steady-state plasma levels were greater than or equal to 10 microM for at least 96 hr, and this schedule demonstrated minimal toxicity. Administration of VCR 20 hr after the start of VRP infusion produced significant lethality, requiring an 8-fold reduction in the VCR dose. Pharmacokinetic studies showed that VRP markedly increased the uptake and retention of VCR in small intestine, liver and kidney of mice. In small intestine, 8-fold greater levels of VCR were determined 24 hr after VCR administration, and this was associated with in increase in T1/2 for elimination from 350 to 913 min. HPLC analysis of extracts from small intestine showed that greater than 90% of the radiolabel eluted with VCR or 4-desacetyl-VCR. Modulation of VCR retention was also related to the dose of VCR administered. The VRP-sensitive efflux pathway appeared more effective in certain tissues only at higher concentrations of VCR. In contrast, VRP did not alter significantly the uptake and retention of VCR in either the parent or VCR-resistant human xenografts. The data demonstrated that, in the mouse, VRP modulates the uptake and retention of VCR in several tissues, and this may indicate that drug efflux mediated by a VRP-sensitive mechanism (e.g. GP-170, associated with the multiple drug resistance phenotype) has a protective function against xenobiotics in these tissues.