NON-P-GLYCOPROTEIN MULTIDRUG-RESISTANCE IN CELL-LINES WHICH ARE DEFECTIVE IN THE CELLULAR ACCUMULATION OF DRUG

Non-Pgp mdr related to a defect in drug accumulation has now been documented in a number of different cell lines exposed to certain cytotoxic agents. In studies conducted thus far most isolates have been obtained after selection in either adriamycin or mitoxantrone. The work in this area is in its early stages and very little is known about the molecular events which contribute to this mode of drug resistance. At the present time no protein with drug binding properties comparable to Pgp has been identified in non-Pgp mdr isolates. Evidence based on the finding that all isolates do not respond in the same way to reversal agents such as verapamil suggests the possibility that more than one mechanism may exist for non-Pgp mdr. Future studies may thus reveal that cells contain a multiplicity of genes which upon transcriptional activation can function to alter drug transport processes and thus contribute to the development of mdr. Identifying and characterizing these genes will be important since they may function in transport systems of normal cells. The exact identity of proteins which contribute to non-Pgp mdr remains to be determined. One protein designated P190 has been found to be overexpressed in cell lines of human promyelocytic leukemia, lung and adenocarcinoma treated with adriamycin. The protein also is increased in some clinical samples from patients undergoing chemotherapy. P190 which has a minor sequence homology with Pgp can bind ATP and may thus contribute to the energy dependent drug efflux systems found in cells containing this protein. Transfection studies with a P190 cDNA should determine whether this protein actually contributes to drug resistance. Many other protein changes have been detected in non-Pgp mdr cells but the importance of these in resistance also remains to be determined. In some systems a particular protein change can be identified in multiple independent isolates suggesting a correlation between the development of resistance and the presence of this cellular alteration. Experiments conducted thus far on the mechanism of non-Pgp mdr are intriguing. Studies utilizing fluorescence microscopy to follow the fate of daunomycin suggests that the drug passes to the interior of the cell and eventually localizes in the Golgi apparatus. Drug located at this site may move directly into an efflux pathway for rapid extrusion from the cell. Evidence also indicates that as drug leaves the Golgi some may be sequestered into other organelles such as lysosomes or mitochondria. Sequestration may thus be another means of protecting the cell from the cytotoxic action of the drug. Very little is known of the molecular details of these events and some new technological approaches may be required to gain insight into efflux and sequestration pathways. In vitro systems for drug transport would certainly be important in these studies. A major question to be answered in the future is whether non-Pgp mdr actually contributes to clinical drug resistance. This will certainly be clarified as new probes which can selectively detect this type of resistance are developed. Some studies have shown that in experimental isolates a low level non-Pgp mdr can precede a Pgp mdr which appears after continuous treatment of cells with drug. Possibly these findings have clinical relevance.