Availability of PSC833, a substrate and inhibitor of P-glycoproteins, in various concentrations of serum.
BACKGROUND
P-glycoproteins are membrane-associated transporters that can render cells resistant to a variety of chemotherapeutic drugs. Reversal agents are (preferably nontoxic) drugs that can inhibit these P-glycoproteins and thereby overcome multidrug resistance. PSC833, a cyclosporin A analog, is a reversal agent that has shown potential in in vitro experiments and in clinical trials. We tested PSC833 to determine whether it is a transported substrate of human and murine P-glycoproteins associated with multidrug resistance (encoded by the human MDR1 gene and its murine homolog, mdr1a) and whether it can completely inhibit these P-glycoproteins under simulated in vivo conditions.


METHODS
Monolayers of polarized LLC-PK1 pig kidney cells transfected with complementary DNA containing either MDR1 or mdr1a sequences were used to measure the directional transport of P-glycoprotein substrates under various serum conditions.


RESULTS
In contrast to two previous studies, we found that PSC833 is transported by both the MDR1 and the mdr1a P-glycoproteins, albeit at a low rate. PSC833 has a very high affinity for the MDR1 P-glycoprotein, and its Michaelis constant (Km) for transport is 50 nM, fourfold lower than for cyclosporin A. Inhibition of drug transport by PSC833 is approximately eightfold less effective in 100% fetal bovine serum than in tissue culture medium containing 10% serum. The concentration of PSC833 necessary to fully inhibit transport of digoxin and paclitaxel (Taxol) under complete (i.e., 100%) serum conditions is higher than the plasma concentrations achieved in clinical trials.


CONCLUSIONS
Although PSC833 binds efficiently to the MDR1 P-glycoprotein and is released only sluggishly, the high concentrations of PSC833 necessary to inhibit this P-glycoprotein under complete serum conditions in our in vitro system suggest that it may be difficult for PSC833 alone to produce total inhibition of P-glycoprotein activity in patients.
Division of Molecular Biology, The Netherlands Cancer Institute, Amsterdam.
A J
AJ
Smith
U
U
Mayer
A H
AH
Schinkel
P
P
Borst
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D000818:Animals; D001682:Biological Availability; D001692:Biological Transport; D002250:Carbon Radioisotopes; D002478:Cells, Cultured; D016895:Culture Media, Serum-Free; D003524:Cyclosporins / Q000097:blood / Q000493:pharmacokinetics*; D006801:Humans; D007668:Kidney / Q000166:cytology / Q000378:metabolism; D051379:Mice; D013552:Swine; D014162:Transfection
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002250:Carbon Radioisotopes; D016895:Culture Media, Serum-Free; D003524:Cyclosporins; C070272:valspodar