Phase I and pharmacokinetic study of lexatumumab (HGS-ETR2) given every 2 weeks in patients with advanced solid tumors.

BACKGROUND: Lexatumumab (HGS-ETR2) is a fully human agonistic mAb to the tumor necrosis factor-related apoptosis-inducing ligand receptor 2 that activates the extrinsic apoptosis pathway and has potent preclinical antitumor activity. MATERIALS AND METHODS: This phase 1, dose escalation study assessed the safety, tolerability, pharmacokinetics (PKs) and immunogenicity of lexatumumab administered i.v. every 14 days in patients with advanced solid tumors. RESULTS: Thirty-one patients received lexatumumab over five dose levels (0.1-10 mg/kg). Most (26 of 31) received four or more cycles of treatment. One patient at 10 mg/kg experienced a possibly related dose-limiting toxicity of grade 3 hyperamylasemia. Nine patients achieved stable disease. One patient with chemotherapy-refractive Hodgkin's disease experienced a mixed response. Lexatumumab PKs were linear up to 10 mg/kg. At the 10 mg/kg dose, the mean (+/-standard deviation) t(1/2b) was 13.67 +/- 4.07 days, clearance was 4.95 +/- 1.93 ml/day/kg, V(1) was 45.55 ml/kg and V(ss) was 79.08 ml/kg, indicating that lexatumumab distributes outside the plasma compartment. No human antihuman antibodies were detected. CONCLUSIONS: Lexatumumab can be safely administered every 14 days at 10 mg/kg. The PK profile supports this schedule. Further evaluation of lexatumumab at this dose schedule is warranted, including combination trials with other agents.

Low or absent SPARC expression in acute myeloid leukemia with MLL rearrangements is associated with sensitivity to growth inhibition by exogenous SPARC protein.

Secreted protein, acidic and rich in cysteine (SPARC), is a matricellular glycoprotein with growth-inhibitory and antiangiogenic functions. Although SPARC has been implicated as a tumor suppressor in humans, its function in normal or malignant hematopoiesis has not previously been studied. We found that the leukemic cells of AML patients with MLL gene rearrangements express low to undetectable amounts of SPARC whereas normal hematopoietic progenitors and most AML patients express this gene. SPARC RNA and protein levels were also low or undetectable in AML cell lines with MLL translocations. Consistent with its tumor suppressive effects in various solid tumor models, exogenous SPARC protein selectively reduced the growth of cell lines with MLL rearrangements by inhibiting cell cycle progression from G1 to S phase. The lack of SPARC expression in MLL-rearranged cell lines was associated with dense promoter methylation. However, we found no evidence of methylation-based silencing of SPARC in primary patient samples. Our results suggest that low or absent SPARC expression is a consistent feature of AML cells with MLL rearrangements and that SPARC may function as a tumor suppressor in this subset of patients. A potential role of exogenous SPARC in the therapy of MLL-rearranged AML warrants further investigation.

A phase I trial of liposomal doxorubicin, paclitaxel and valspodar (PSC-833), an inhibitor of multidrug resistance.

PURPOSE: The aim of this study was to determine (i) the maximum tolerated dose (MTD) of liposomal doxorubicin (L-DOX) and paclitaxel (DP), (ii) the MTD of DP plus valspodar (DPV) and (iii) pharmacokinetic (PK) interactions of valspodar with L-DOX and paclitaxel. METHODS: Twenty-three patients with metastatic cancers received DP, followed 4 weeks later by DPV. Dose levels of DP were (mg/m2 for L-DOX/paclitaxel): 30/135 (n = 7), 30/150 (n = 4), 35/150 (n = 8) and 40/150 (n = 4). Dose levels of DPV were 15/70 (n = 10) and 15/60 (n = 10). Serial, paired PK studies were performed. RESULTS: The MTD of DP was 40/150. For DPV at 15/70, five of 10 patients experienced grade 4 neutropenia. In the next cohort, a reduced dose of 15/60 was well tolerated. Valspodar produced reversible grade 3 ataxia in seven patients, requiring dose reduction from 5 to 4 mg/kg. Paired PK studies indicated no interaction between L-DOX and valspodar, and a 49% increase in the median half-life of paclitaxel. Two partial and one minor remissions were noted. CONCLUSIONS: The use of valspodar necessitated dose reductions of DP, with neutropenia being dose limiting. Valspodar PK interactions were observed with paclitaxel but not L-DOX.

Pharmacokinetic interactions of cyclosporine with etoposide and mitoxantrone in children with acute myeloid leukemia.

The purpose of this study was to assess the effect of the multidrug resistance modulator cyclosporine (CsA) on the pharmacokinetics of etoposide and mitoxantrone in children with de novo acute myeloid leukemia (AML). Serial blood samples for pharmacokinetic studies were obtained in 38 children over a 24-h period following cytotoxin treatment with or without CsA on days 1 and 4. Drug concentrations were quantitated using validated HPLC methods, and pharmacokinetic parameters were determined using compartmental modeling with an iterative two-stage approach, implemented on ADAPT II software. Etoposide displayed a greater degree of interindividual variability in clearance and systemic exposure than mitoxantrone. With CsA treatment, etoposide and mitoxantrone mean clearance declined by 71% and 42%, respectively. These effects on clearance, in combination with the empiric 40% dose reduction for either cytotoxin, resulted in a 47% and 12% increases in the mean AUC for etoposide and mitoxantrone, respectively. There were no differences in the rates of stomatitis or infection between the two groups. CsA treatment resulted in an increased incidence of hyperbilrubinemia, which rapidly reversed upon conclusion of drug therapy. The variability observed in clearance, combined with the empiric 40% dose reduction of the cytotoxins, resulted in statistically similar systemic exposure and similar toxicity.

A phase I trial of doxorubicin, paclitaxel, and valspodar (PSC 833), a modulator of multidrug resistance.

PURPOSE: P-glycoprotein is an efflux pump for many drugs including doxorubicin and paclitaxel. This study evaluated the coadministration of these drugs with the P-glycoprotein inhibitor valspodar (PSC 833) with the aim of determining: (a) maximum tolerated doses (MTDs) of doxorubicin followed by paclitaxel (DP); (b) the MTD of DP combined with PSC 833 (DPV), without and with filgrastim (G-CSF); and (c) the pharmacokinetic interactions of PSC 833 with doxorubicin and paclitaxel. EXPERIMENTAL DESIGN: For the first cycle, patients received doxorubicin as a 15-min infusion followed by paclitaxel as a 1-h infusion. For the second cycle, patients received reduced doses of DP with PSC 833 at 5 mg/kg p.o., four times a day for 12 doses. RESULTS: Thirty-three patients with various refractory malignancies were enrolled and assessable. The MTD of DP without PSC 833 was 35 mg/m(2) doxorubicin and 150 mg/m(2) paclitaxel. The MTD of DPV without G-CSF was 12.5 mg/m(2) doxorubicin and 70 mg/m(2) paclitaxel. The dose-limiting toxicity for both DP and DPV was neutropenia without thrombocytopenia. With G-CSF, the MTD for DPV was 20 mg/m(2) doxorubicin and 90 mg/m(2) paclitaxel. No grade 4 nonhematological toxicities were observed. Five partial and two minor tumor remissions were observed. Paired pharmacokinetics with and without PSC 833 revealed substantial drug interactions with both doxorubicin and paclitaxel. CONCLUSIONS: PSC 833 can be administered safely with doxorubicin and paclitaxel. The pharmacokinetic profiles of these drugs are significantly affected by PSC 833, requiring approximately 60% dose reductions for equivalent degrees of myelosuppression.

MDR 1 activation is the predominant resistance mechanism selected by vinblastine in MES-SA cells.

Single-step selection with vinblastine was performed in populations of the human sarcoma cell line MES-SA, to assess cellular mechanisms of resistance to the drug and mutation rates via fluctuation analysis. At a stringent selection with 20 nM vinblastine, resulting in 5-6 logs of cell killing, the mutation rate was 7 x 10(-7)per cell generation. Analysis of variance supported the hypothesis of spontaneous mutations conferring vinblastine resistance, rather than induction of adaptive response elements. Surviving clones displayed a stable multidrug resistance phenotype over a 3-month period. All propagated clones demonstrated high levels of resistance to vinblastine and paclitaxel, and lower cross-resistance to doxorubicin and etoposide. Activation of MDR 1 gene expression and P-glycoprotein function was demonstrable in all clones. No elevation was found in the expression of the mrp gene, the LRP-56 major vault protein and beta-tubulin isotypes (M40, beta4, 5beta, and beta9) in these mutants. We conclude that initial-step resistant mechanism in these vinblastine-selected mutants commonly arises from a stochastic mutation event with activation of the MDR 1 gene.

Decreased cortisol secretion by adrenal glands perfused with the P-glycoprotein inhibitor valspodar and mitotane or doxorubicin.

The aim of this study was to investigate the role of P-glycoprotein (P-gp) in the adrenal gland. It has been presumed that P-gp, rather than being involved in physiological cortisol secretion, plays a role in protecting the adrenacortical cells from xenobiotics. To explore this a study was performed on perfused bovine adrenal glands. Individual experimental groups were perfused with either a selective P-gp blocker (valspodar) alone, with a xenobiotic (mitotane or doxorubicin) alone or with both valspodar and a xenobiotic. The cumulative amounts of cortisol secreted in each individual group were calculated and the two-sample t-test was used to compare the mean values of cumulative amounts. The mean value of cortisol secreted from the group of adrenals perfused with the P-gp blocker was not significantly different from that of the control group. Treatment with either mitotane or doxorubicin decreased the amount of cortisol secreted but not significantly when compared to the amount of cortisol secreted in basal conditions. However, treatment with the P-gp blocker valspodar in addition to either mitotane or doxorubicin significantly decreased cortisol secreted compared to the amount of cortisol secreted by the glands treated with either mitotane (p=0.009) or doxorubicin (p=0.017) alone. The regressive changes discovered in all experimental groups were most prominent when valspodar was used with either mitotane or doxorubicin. We found that P-gp blockade increases by xenobiotic (mitotane and doxorubicin)-induced damage of adrenocortical cells, which points to a role of P-gp in the protection of adrenal gland from xenobiotics.

Clinical studies of antisense therapy in cancer.

The ability to target and inhibit individual gene expression with antisense oligonucleotides has shown promising activity in preclinical cancer models. Recent clinical studies have tested antisense compounds directed against seven cancer related genes including p53, bcl-2, c-raf, H-ras, protein kinase C-alpha, and protein kinase A. Class specific effects of the phosphorothioate backbone common to the first generation of antisense compounds have dominated the side effects of these oligonucleotides. Inhibition of target gene expression has been modest at most, and clinical activity has been primarily anecdotal. Combinations of the antisense compounds with chemotherapy and second-generation oligonucleotides offer promise that these agents might become a standard part of future cancer therapy.

Mitoxantrone, etoposide, and cyclosporine therapy in pediatric patients with recurrent or refractory acute myeloid leukemia.

PURPOSE: To determine the remission rate and toxicity of mitoxantrone, etoposide, and cyclosporine (MEC) therapy, multidrug resistance-1 (MDR1) status, and steady-state cyclosporine (CSA) levels in children with relapsed and/or refractory acute myeloid leukemia. PATIENTS AND METHODS: MEC therapy consisted of mitoxantrone 6 mg/m(2)/d for 5 days, etoposide 60 mg/m(2)/d for 5 days, and CSA 10 mg/kg for 2 hours followed by 30 mg/kg/d as a continuous infusion for 98 hours. Because of pharmacokinetic interactions, drug doses were decreased to 60% of those found to be effective without coadministration of CSA. MDR1 expression was evaluated by reverse transcriptase polymerase chain reaction, flow cytometry, and the ability of CSA at 2.5 micromol/L to increase intracellular accumulation of (3)H-daunomycin in blasts from bone marrow specimens. RESULTS: The remission rate was 35% (n = 23 of 66). Overall, 35% of patients (n = 23) achieved complete remission (CR), 12% of patients (n = 8) achieved partial remission, and 9% of patients (n = 6) died of infection. Exposure to CSA levels of greater than 2,400 ng/mL was achieved in 95% of patients (n = 56 of 59). Toxicities included infection, cardiotoxicity, myelosuppression, stomatitis, and reversible increases in serum creatinine and bilirubin. In most who had relapsed while receiving therapy or whose induction therapy had failed, response was not significantly different for MDR1-positive and MDR1-negative patients. CONCLUSION: Serum levels of CSA capable of reversing multidrug resistance are achievable in children with acceptable toxicity. The CR rate of 35% achieved in this study is comparable to previously reported results using standard doses of mitoxantrone and etoposide. The use of CSA may have improved the response rate for the MDR1-positive patients so that it was not different from that for the MDR1-negative patients.

Loss of cyclosporin and azidopine binding are associated with altered ATPase activity by a mutant P-glycoprotein with deleted phe(335).

In this study, we further characterize a mutant P-glycoprotein (P-gp) that has a deletion of Phe(335) and is resistant to inhibition by cyclosporins. Photoaffinity labeling with [(3)H]cyclosporine and [(3)H]azidopine revealed markedly decreased binding to the mutant P-gp compared with wild-type P-gp. Expression of the mutant P-gp in multidrug-resistant variant cell line MES-SA/DxP (DxP) cells was associated with a 2-fold higher basal ATPase activity relative to multidrug-resistant cell line MES-SA/Dx5 (Dx5) cells with wild-type P-gp. Cyclosporine inhibited ATPase activity in both cell types, whereas the cyclosporin D analog valspodar (PSC 833), vinblastine, and dactinomycin stimulated ATPase activity in Dx5 but not in mutant DxP cells. Moreover, the cell lines differed in their responses to verapamil, which produced greater stimulation of ATPase in Dx5 than DxP cells. Verapamil significantly reversed the [(3)H]daunorubicin accumulation defect in wild-type Dx5 cells, but it had no significant effect on [(3)H]daunorubicin accumulation in the mutant DxP cells. Verapamil was not transported by cells expressing either mutant or wild-type P-gp. Vanadate trapping of azido-ATP was markedly impaired in mutant P-gp. In conclusion, our data demonstrate that Phe(335) of transmembrane 6 is an important amino acid residue for the formation of cyclosporine and azidopine drug-binding site(s). Phe(335) also plays a role in the coupling of verapamil binding and modulation of daunorubicin intracellular accumulation in wild-type P-gp. In addition, Phe(335) in transmembrane 6 may play a role in coupling drug binding to ATPase activity. The deletion of Phe(335) results in a significant increase in the basal ATPase activity with a concomitant decrease in its ability to trap ATP and transport some P-gp substrates.

Effect of high-dose cyclosporine on etoposide pharmacodynamics in a trial to reverse P-glycoprotein (MDR1 gene) mediated drug resistance.

PURPOSE: The consequences of using cyclosporine (CsA) therapy to modulate P-glycoprotein-mediated multidrug resistance include increased myelosuppression, hyperbilirubinemia, and altered disposition of the cytotoxin. The purpose of this study was to analyze further the relationship between the degree of leukopenia, and etoposide pharmacokinetic factors. METHODS: Each patient initially received intravenously-administered etoposide alone (150-200 mg/m2/d x 3). Later it was given in combination with CsA administered at escalating loading doses (range 2-7 mg/kg) as a 2 hour intravenous (IV) infusion followed by a 3 day continuous infusion, at doses ranging from 5 to 21 mg/ kg/day. Serial plasma etoposide concentration-time samples were assayed by high-performance liquid chromatography (HPLC). The area under the curve (AUC) of unbound etoposide was calculated from the total plasma etoposide AUC using a previous published equation [22] where % unbound etoposide = (1.4 x total bilirubin) - (6.8 x serum albumin) + 34.4. The percent decrease in white blood cell (WBC) count and the total or unbound etoposide AUC relationship was fitted to a sigmoid Emax model adapted for paired observations, where: % Decrease in WBC count =E(max) x PDRV(H+Z x delta)/(PDRV50 + Z x beta) + PDRVH + Z x delta In this equation, Z was the variable describing the two treatment groups (0 = no CsA and 1 = CsA). The fitted parameters were PDRV50, the pharmacodynamic response variable (PDRV) producing 50% of the maximal response; parameter beta, which describes the effect of the treatment group on the PDRV50; parameter H (Hill constant), which defines the slope of the response curve and parameter delta, which describes the effect of the treatment group on parameter H. RESULTS: CsA at a median concentration of 1,938 microg/ml resulted in a median increase in the total plasma etoposide AUC by 103% and the calculated unbound plasma etoposide AUC by 104%. This paralleled a 12% greater median percent decrease in WBC count during etoposide + CsA treatment (72% vs. 84%, P = 0.03). The percent decrease in WBC count and total or unbound etoposide AUC relationship was fitted to the sigmoid Emax model. The model using the unbound etoposide AUC described the data adequately (r = 0.790) and was precise, with a mean absolute error of 6.4% (95% confidence interval: -4.9, 7.8). The fitted parameter-estimates suggested that at equivalent unbound etoposide AUC values above 10 microg x h/ml, the sigmoid Emax model predicted a 5% greater WBC count suppression when CsA was added to the treatment regimen. CONCLUSION: These findings suggest that a small degree of the enhanced myelosuppression observed with CsA combined with etoposide might be attributable to inhibition of P-glycoprotein in bone marrow precursor cells. However, the majority of the effect observed appears to be due to pharmacokinetic interactions, which result in increases in unbound etoposide.

Phase I study of an antisense oligonucleotide to protein kinase C-alpha (ISIS 3521/CGP 64128A) in patients with cancer.

Protein kinase C (PKC) is an attractive target in cancer therapy. It is overexpressed in a variety of cancers, and nonspecific inhibitors of PKC have demonstrated antitumor activity. Antisense oligonucleotides targeted against PKC-alpha, which have high specificity, can inhibit mRNA and protein expression as well as the growth of tumors in vitro and in vivo. This Phase I study sought to characterize the safety profile and to determine the maximum tolerated dose of antisense to PKC-alpha when administered by continuous infusion in patients. Patients with incurable malignancies received ISIS 3521, a 20-length phosphorothioate oligodeoxynucleotide specific for PKC-alpha. Treatment was delivered over a period of 21 days by continuous i.v. infusion followed by a 7-day rest period. Doses were increased from 0.5 to 3.0 mg/kg/day. Patients continued on the study until evidence of disease progression or unacceptable toxicity was detected. Between August 1996 and September 1997, 21 patients were treated in five patient cohorts. The maximum tolerated dose was 2.0 mg/kg/day. The dose-limiting toxicities were thrombocytopenia and fatigue at a dose of 3.0 mg/kg/day. Pharmacokinetic measurements showed rapid plasma clearance and dose-dependent steady-state concentrations of ISIS 3521. Evidence of tumor response lasting up to 11 months was observed in three of four patients with ovarian cancer. The recommended dose of ISIS 3521 for Phase II studies is 2.0 mg/kg/day when given over a period of 21 days. Side effects are modest and consist of thrombocytopenia and fatigue. Evidence of antitumor activity provides the rationale for Phase II studies in ovarian cancer and other malignancies.

Modulation of multidrug resistance: a paradigm for translational clinical research.

Resistance of cancer cells is the major limitation to the success of chemotherapy. Although many mechanisms of cellular resistance to anticancer drugs have been defined, the best understood of these is multidrug resistance (MDR), caused by the multidrug transporter, P-glycoprotein (P-gp), the product of the MDR1 gene. New drugs developed specifically to inhibit P-gp and modulate MDR, such as valspodar (PSC 833 [Amdray]), are currently undergoing clinical testing. Moreover, agents designed to inhibit other mechanisms of drug resistance are currently in development, and concurrent blockade of multiple mechanisms of resistance appears to be a promising approach. Coadministration of MDR1-related chemotherapeutic drugs with an MDR modulator may enhance the bioavailability of these agents sufficiently to enable oral dosing, which would potentially be more convenient and less toxic.

Mechanisms of action of and resistance to antitubulin agents: microtubule dynamics, drug transport, and cell death.

PURPOSE: To analyze the available data concerning mechanisms of action of and mechanisms of resistance to the antitubulin agents, vinca alkaloids and taxanes, and more recently described compounds. DESIGN: We conducted a review of the literature on classic and recent antitubulin agents, focusing particularly on the relationships between antitubulin agents and their intracellular target, the soluble tubulin/microtubule complex. RESULTS AND CONCLUSION: Although it is widely accepted that antitubulin agents block cell division by inhibition of the mitotic spindle, the mechanism of action of antitubulin agents on microtubules remains to be determined. The classic approach is that vinca alkaloids depolymerize microtubules, thereby increasing the soluble tubulin pool, whereas taxanes stabilize microtubules and increase the microtubular mass. More recent data suggest that both classes of agents have a similar mechanism of action, involving the inhibition of microtubule dynamics. These data suggest that vinca alkaloids and taxanes may act synergistically as antitumor agents and may be administered as combination chemotherapy in the clinic. However, enhanced myeloid and neurologic toxicity, as well as a strong dependence on the sequence of administration, presently exclude these combinations outside the context of clinical trials. Although the multidrug resistance phenotype mediated by Pgp appears to be an important mechanism of resistance to these agents, alterations of microtubule structure resulting in altered microtubule dynamics and/or altered binding of antitubulin agents may constitute a significant mechanism of drug resistance.

Mitoxantrone, etoposide, and cytarabine plus cyclosporine for patients with relapsed or refractory acute myeloid leukemia: an Eastern Cooperative Oncology Group pilot study.

BACKGROUND: One potential mechanism of drug resistance to chemotherapy is the overexpression of multidrug resistance (MDR) genes coding for P-glycoprotein (P-gp), which leads to reduced intracellular retention of chemotherapy. This study tested the efficacy and toxicity of mitoxantrone, etoposide, and intermediate dose cytarabine (MEC) with cyclosporine (CSP) as an MDR modulator in patients with recurrent and refractory acute myeloid leukemia, and also correlated P-gp expression in leukemia cells with response. METHODS: Thirty-eight eligible patients who were in first recurrence after < 6 months of complete remission (CR) (11 patients), refractory to initial induction therapy or to one attempt at reinduction after recurrence (18 patients), in second recurrence (4 patients), or in recurrence after either allogeneic or autologous bone marrow transplantation (5 patients) received either MEC alone (13 patients) or MEC-CSP (25 patients). CSP was given as a loading dose of 6 mg/kg for 2 hours intravenously (i.v.) starting 2 hours before the first dose of etoposide, followed by a continuous i.v. infusion of 18 mg/kg/day for 98 hours. RESULTS: Three of the 13 patients (23%) who received MEC achieved CR, as did 6 of the 25 patients (24%) who received MEC-CSP. The median remission duration for all patients who achieved CR was 149 days (range, 26-466 days), 91 days (range, 81-172 days) for the 3 patients who received MEC, and 189.5 days (range, 26-466 days) for the patients treated with MEC-CSP. The median survival for the patients treated with MEC and MEC-CSP was 104 and 72 days, respectively. CONCLUSIONS: No significant association was found between P-gp expression and response. No apparent benefit in the CR rate, remission duration, or survival was observed with the addition of CSP to MEC.

Treatment of refractory and relapsed acute myelogenous leukemia with combination chemotherapy plus the multidrug resistance modulator PSC 833 (Valspodar).

A potential mechanism of chemotherapy resistance in acute myeloid leukemia (AML) is the multidrug resistance (MDR-1) gene product P-glycoprotein (P-gp), which is often overexpressed in myeloblasts from refractory or relapsed AML. In a multicenter phase II clinical trial, 37 patients with these poor risk forms of AML were treated with PSC 833 (Valspodar; Novartis Pharmaceutical Corporation, East Hanover, NJ), a potent inhibitor of the MDR-1 efflux pump, plus mitoxantrone, etoposide, and cytarabine (PSC-MEC). Pharmacokinetic (PK) interactions of etoposide and mitoxantrone with PSC were anticipated, measured in comparison with historical controls without PSC, and showed a 57% decrease in etoposide clearance (P =.001) and a 1.8-fold longer beta half-life for mitoxantrone in plasma (P <.05). The doses of mitoxantrone and etoposide were substantially reduced to compensate for these interactions and clinical toxicity and in Cohort II were well tolerated at dose levels of 4 mg/m2 mitoxantrone, 40 mg/m2 etoposide, and 1 g/m2 C daily for 5 days. Overall, postchemotherapy marrow hypoplasia was achieved in 33 patients. Twelve patients (32%) achieved complete remission, four achieved partial remission, and 21 failed therapy. The PK observations correlated with enhanced toxicity. The probability of an infectious early death was 36% (4 of 11) in patients with high PK parameters for either drug versus 5% (1 of 20) in those with lower PK parameters (P =.04). P-gp function was assessed in 19 patients using rhodamine-123 efflux and its inhibition by PSC. The median percentage of blasts expressing P-gp was increased (49%) for leukemic cells with PSC-inhibitable rhodamine efflux compared with 17% in cases lacking PSC-inhibitable efflux (P =.004). PSC-MEC was relatively well tolerated in these patients with poor-risk AML, and had encouraging antileukemic effects. The Eastern Cooperative Oncology Group is currently testing this regimen versus standard MEC chemotherapy in a phase III trial, E2995, in a similar patient population.

New approaches in cancer treatment.

Major advances in cellular biology, genetics, pharmacology and immunology in the past decade are beginning to be translated into progress in cancer treatment. This progress is manifested by new cytotoxic drugs which have recently entered clinical practice (taxanes, topoisomerase I inhibitors, gemcitabine, vinorelbine, new purines), as well as the efficacy of monoclonal antibody therapies against the CD-20 antigen of B-cell lymphomas and the Her2/neu oncogene in breast cancer. Several new drugs in development are targeted at reversal or prevention of the multidrug resistance mechanism caused by expression of the MDR1 gene (P-glycoprotein). Tumour angiogenesis as a target is being studied in several early clinical trials. As with many other biological therapies, the evaluation of these compounds and their integration with standard therapies presents a major challenge to clinical investigators. The emerging field of genomics and gene expression micro-arrays will provide enormous information about the biology of cancers. This technology offers great opportunities for the discovery of new therapeutic targets, which should provide a basis for the design and evaluation of many new agents in the coming decade.

Interaction of anti-HIV protease inhibitors with the multidrug transporter P-glycoprotein (P-gp) in human cultured cells.

The anti-HIV protease inhibitors represent a new class of agents for treatment of HIV infection. Saquinavir, ritonavir, indinavir, and nelfinavir are the first drugs approved in this class and significantly reduce HIV RNA copy number with minimal adverse effects. They are all substrates of cytochrome P450 3A4, and are incompletely bioavailable. The drug transporting protein, P-glycoprotein (P-gp), which is highly expressed in the intestinal mucosa, could be responsible for the low oral bioavailability of these and other drugs which are substrates for this transporter. To determine whether these protease inhibitors are modulators of P-gp, we studied them in cell lines which do and do not express P-gp. Saquinavir, ritonavir and nelfinavir significantly inhibited the efflux of [3H]paclitaxel and [3H]vinblastine in P-gp-positive cells, resulting in an increase in intracellular accumulation of these drugs. However, similar concentrations of indinavir did not affect the accumulation of these anticancer agents. In photoaffinity labeling studies, saquinavir and ritonavir displaced [3H]azidopine, a substrate for P-gp, in a dose-dependent manner. These data suggest that saquinavir, ritonavir, and nelfinavir are inhibitors and possibly substrates of P-gp. Because saquinavir has a low bioavailability, its interaction with P-gp may be involved in limiting its absorption.

Effect of the multidrug resistance modulator valspodar on serum cortisol levels in rabbits.

PURPOSE: To contribute to a better understanding of the physiological role of P-glycoprotein (P-gp) in the adrenal gland, we initiated our studies in rabbits. The aim of our study was to explore the effect of the selective multidrug resistance (MDR) modulator PSC 833 (valspodar) on serum cortisol in rabbits. METHODS: Baseline and corticotropin-stimulated serum cortisol levels were measured before and after valspodar treatment in adult male rabbits. Seven rabbits were treated with 50 mg/kg per dose and seven, with 75 mg/kg per dose of valspodar subcutaneously. Serum cortisol levels were determined by radioimmunoassay adjusted for expected values. RESULTS: Serum cortisol levels (baseline as well as corticotropin-stimulated) increased after both valspodar treatment regimens. The increase was dose-dependent and was higher for the baseline than for the corticotropin-stimulated values. Serum valspodar levels exceeding 1000 ng/ml were achieved in all except one animal in each group. We hypothesize that the increased serum cortisol levels were due to increased adrenocorticotropic hormone (ACTH) secretion after valspodar treatment, but, unfortunately, we could not measure ACTH properly in rabbits by means of the commercially available kits. CONCLUSIONS: Our study indicates that P-gp is not involved in steroid hormone secretion in the adrenal gland. This is evident from observations that serum cortisol levels were found to have increased rather than decreased in rabbits treated with a P-gp blocker and that the treated animals appeared healthy and normal. Since P-gp was found to play an important role in protection against xenobiotics in some other organs, further studies to explore the protective role of P-gp in the adrenal gland are warranted.

Pharmacologic approaches to reversing multidrug resistance.

The rationale for modulation of multidrug resistance (MDR) by inhibitors of the multidrug transporter, P-glycoprotein (P-gp) includes the following: (1) P-gp is expressed by human cancers, either at diagnosis or after failure of chemotherapy; (2) P-gp expression at diagnosis has been associated with a poor prognosis in some types of cancer; (3) MDR related to P-gp expression can be reversed by modulators, resulting in enhanced therapeutic efficacy in cellular and animal models of drug resistance; and (4) the emergence of MDR related to P-gp expression can be prevented in cellular models by co-administration of MDR-related cytotoxins and modulators. Clinical trials of modulation of MDR have been limited by two major factors: inability to achieve adequate levels of the modulators to reverse drug resistance in patients and the presence of other mechanisms of resistance in tumor cells in addition to P-gp. The former limitation will hopefully be overcome by new, more potent and specific inhibitors of P-gp such as PSC 833. The latter will require further understanding of various alternative cellular mechanisms of resistance and the development of approaches to overcome or circumvent these mechanisms. PSC 833 is associated with significant drug interactions with MDR-related cytotoxic agents, which require dose reduction of the cytotoxins to achieve a dose exposure and toxicity similar to the chemotherapy agents without a modulator. These drug interactions are predictable and are at least in part due to inhibition of P-gp in normal tissues such as the liver and kidneys, where P-gp is known to play a role in drug excretion. Data from knockout mice, which lack P-gp expression, support the concept that P-gp is an important factor in MDR-related drug disposition. Early data from phase I and II trials with PSC 833 indicate that substantial inhibition of P-gp can be achieved in patients at clinically tolerable doses of both modulator and cytotoxins. The ultimate therapeutic benefit of MDR modulation with PSC 833 is currently being tested in phase III clinical trials in acute myeloid leukemias (AMLs) and multiple myeloma.