Multidrug resistance in cancer

Malignant neoplasms vary in their response to cytotoxic drugs: some are sensitive, others resistant. Understanding of the biochemical basis of this resistance might lead to the development of markers that would correlate with the clinical response to the drugs - or even lead to ways of overcoming the resistance.

Multidrug resistance was first described in 1970 after selection of Chinese hamster ovarian cancer cells exposed to increasing concentrations of actinomycin D.1 Though the cells had been selected by a single agent, they proved to be resistant to a range of clinically important anticancer drugs, including the anthracyclines (doxorubicin and daunomycin), the vinca alkaloids (vincristine, vinblastine, and vindesine), etoposide, and colchicine. Riordan and Ling went on to show that the multidrug resistant cells had lower concentrations of the drug - a drug accumulation deficit - and that a membrane glycoprotein of 170 kDa was responsible.2 At first the deficit was thought to be due to a fault in permeation and the glycoprotein was named P glycoprotein (P-gp).3 The gene for this, mdr-1, has been cloned and sequenced, and the amino acid has been sequenced and the amino acid sequence derived.⁴ P glycoprotein has 1280 amino acids, and its structure suggests that it arose from the fusion of two closely related genes; it uses ATP as a source of energy. The precise molecular mechanism whereby P glycoprotein can transport such a wide range of structurally diverse drugs is uncertain, and several models have been proposed.*RF 5-7*

P glycoprotein belongs to a superfamily of ATP binding cassette transporters, whose members include the cystic fibrosis transmembrane regulator, the chloroquine transporter of Plasmodium falciparum, and a yeast transporter.8 Recently a new member of this family was cloned from a human small cell lung cancer cell line, H69AR. Though it was resistant to doxorubicin and vincristine, it did not express P glycoprotein.9 This protein has 1522 amino acids and has been termed multidrug resistance associated protein (MRP). Cells with this type of resistance may not have accumulation deficits of doxorubicin. One possible explanation is that multidrug resistance associated protein facilitates the sequestration of cytotoxic drugs into intracellular organelles.9 Whether this protein binds to the drugs against which it confers resistance is not known, but the emergence of a potential new target for modulation, if it is widely expressed in human tumours, is clearly an exciting development.

Several probes have been developed that can detect the P glycoprotein gene, its mRNA, and its protein product.^10,11 Studies have shown that the gene expressed in normal gastrointestinal mucosal cells, renal tubular cells, biliary canalicular cells, and adrenocortical cells.11 Tumours that are initially sensitive to chemotherapy but resistant on relapse commonly show increases in expression of P glycoprotein. Leukaemias with high concentrations of P glycoprotein tend to be resistant to inductive treatment with chemotherapy based on anthracycline.12 Expression of P glycoprotein is an adverse factor in multivariate prognostic models for childhood sarcoma and neuroblastoma.^13,14 Before long it may be practicable to “tissue type” tumours according to expression of P glycoprotein and, perhaps, to avoid the toxicity and reduced quality of life associated with treatment with ineffective anticancer drugs.

After an initial observation by Tsuruo et al,15 several groups have shown that transport defect mediated by P glycoprotein can be blocked by many non-cytotoxic drugs, including nifedipine, verapamil, quinine, chloroquine, progestogens, tamoxifen, cyclosporin A and its analogues, reserpine and tricyclic anti-depressants.16 Clinical trials have assessed the combination of conventional chemotherapy with modulators of P glycoprotein; in one study of relapsed non-Hodgkin's lymphoma 13 of 18 patients responded to infusional chemotherapy with high doses of verapamil.17 Unfortunately, the amounts of verapamil needed to reverse drug resistance produced congestive cardiac failure and heart block. Research groups are currently looking for potent, non-toxic modulators of P glycoprotein to combine with conventional chemotherapy. Cyclosporin A has been reported to reverse clinical multidrug resistance in myeloma.18 The combination of quinidine with the antineoplastic agent epirubicin was recently assessed in a prospective, placebo controlled randomised study in patients with advanced breast cancer.19 Tumour response rates and survival were similar in the two arms of the trial, a finding that probably reflects the low potency of quinidine to bind P glycoprotein.7

Another possible application of the genetics of P glycoprotein is in gene therapy. Transgenic mice that express human P glycoprotein in their bone marrow are resistant to chemotherapy,20 so in theory if human bone marrow could be transfected with the gene that might protect it against myelosuppression from anticancer chemotherapy.8

Novel, improved modulators of P glycoprotein seem likely to be developed in parallel with mechanistic research on the function of the gene, and thus should lead to the rational design of inhibitor. Future phase II clinical trials with new modulators of P glycoprotein must include pharmacokinetic studies since interactions can occur with cytotoxic drugs - for example, etoposide's activity is increased by 80% when it is given in combination with cyclosporin A.21 These trials should focus on those cancers that are known to express high concentrations of P glycoprotein (including renal and colorectal cancers and sarcomas). Since P glycoprotein is expressed in normal tissues there is some concern that its modulators might alter the cellular transport of physiological metabolites such as bilirubin and thereby delay the clearance of cytotoxic drugs that undergo hepatobiliary excretion. These clinical trials should therefore be performed in units used to handling the toxicity associated with intensive chemotherapy.