Abstract
P-glycoprotein (Pgp) is one of the active efflux pumps that are able to extrude a large variety of chemotherapeutic drugs from the cells, causing multidrug resistance. The conformation-sensitive UIC2 monoclonal antibody potentially inhibits Pgp-mediated substrate transport. However, this inhibition is usually partial, and its extent is variable because UIC2 binds only to 10 to 40% Pgp present in the cell membrane. The rest of the Pgp molecules become recognized by this antibody only in the presence of certain substrates or modulators, including vinblastine, cyclosporine A (CsA), and SDZ PSC 833 (valspodar). Simultaneous application of any of these modulators and UIC2, followed by the removal of the modulator, results in a completely restored steady-state accumulation of various Pgp substrates (calcein-AM, daunorubicin, and 99mTc-hexakis-2-methoxybutylisonitrile), indicating near 100% inhibition of pump activity. Remarkably, the inhibitory binding of the antibody is brought about by coincubation with concentrations of CsA or SDZ PSC 833 ∼20 times lower than what is necessary for Pgp inhibition when the modulators are applied alone. The feasibility of such a combinative treatment for in vivo multidrug resistance reversal was substantiated by the dramatic increase of daunorubicin accumulation in xenotransplanted Pgp+ tumors in response to a combined treatment with UIC2 and CsA, both administered at doses ineffective when applied alone. These observations establish the combined application of a class of modulators used at low concentrations and of the UIC2 antibody as a novel, specific, and effective way of blocking Pgp function in vivo.
Footnotes
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This work was supported by OTKA funding TO48742 and TO46945, as well as by NKFP 1A/041/04, GVOP-3.1.1-2004-05-0440/3.0, and GVOP-3.2.1-2004-04-0351/3.0. This publication was also sponsored by the research grant of the Ministry of Public Health ETT 189/2006.
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K.G. and Z.B. are recipients of Bolyai fellowship from the Hungarian Academy of Sciences.
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K.G. and F.F. contributed equally to this work.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.106.110155.
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ABBREVIATIONS: ABC, ATP binding cassette; Pgp, P-glycoprotein; mdr, multidrug resistance; CsA, cyclosporine A; mAb, monoclonal antibody(ies); ACT, antibody competition test; DMEM, Dulbecco's modified Eagle's medium; MIBI, hexakis-2-methoxybutylisonitrile; PI, propidium iodide; GAMIG, goat anti-mouse IgG; PBS, phosphate-buffered saline; BSA, bovine serum albumin; SDZ PSC 833, valspodar; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium; SCID, severe combined immunodeficient; FK506, tacrolimus; XR9576, tariquidar; SR33557, fantofarone.
- Received June 30, 2006.
- Accepted October 17, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
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