CORRESPONDENCE

Re: Folylpolyglutamyl Synthetase Gene Transfer and Glioma Antifolate Sensitivity in Culture and In Vivo

Gerrit Jansen, Godefridus J. Peters, Herbert M. Pinedo, David G. Priest, Yehuda G. Assaraf

Affiliations of authors: G. Jansen, G. J. Peters, and H. M. Pinedo, Department of Medical Oncology, University Hospital Vrije Universiteit, Amsterdam, The Netherlands; D. G. Priest, Department of Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston; Y. G. Assaraf, Department of Biology, The Technion-Israel Institute of Technology, Haifa.

Correspondence to: Gerrit Jansen, Ph.D., Department of Medical Oncology, BR232, University Hospital Vrije Universiteit, De Boelelaan 1117, 1081 HV Amsterdam, The Netherlands (e-mail: g.jansen{at}azvu.nl).

The Journal recently published a report by Aghi et al. (1) and an editorial by Sirotnak (2) about the prospect for folylpolyglutamate synthetase (FPGS) gene transfer as a potentially novel strategy to enhance the therapeutic efficacy of antifolates against solid tumors. This proof of concept was demonstrated by a markedly enhanced therapeutic activity of two polyglutamatable antifolate inhibitors of dihydrofolate reductase, methotrexate and edatrexate, against human glioma cells stably transfected with human FPGS complementary DNA.

We agree with the authors that the outcome of this interesting study deserves further preclinical evaluation. Beyond this, however, we would like to address one important issue that has not received attention in the report by Aghi et al. (1). It is evident that when FPGS gene transfer enhances polyglutamylation of antifolate compounds, it does so as effectively as with natural reduced folate cofactors. Consequently, FPGS gene transfer will enhance the retention of natural folate cofactors and thus alter cellular folate homeostasis by expanding intracellular folate pools. These enhanced folate pools may diminish or even abolish polyglutamylation of antifolates.

This leads us to ask: Have natural reduced-folate cofactor pools been measured in the glioma cells in culture or in vivo after FPGS gene transfer? Recent evidence from preclinical (3-6) and clinical (7) studies indicated that an expansion of the intracellular folate pools can be a major factor in abolishing the activity of antifolates, in particular for antifolates that are highly dependent on polyglutamylation to exert their potent inhibitory effects. For instance, clinically active antifolates, such as ZD1694 (TomudexTM, raltitrexed) and MTA (multitargeted antifolate, LY231514), are marginally active as monoglutamate forms and require polyglutamylation to become biologically active. Impairment of polyglutamylation, in this case by direct competition of antifolates with the expanded folate pools for polyglutamylation by FPGS, is a well-known mechanism of resistance to antifolates. Our studies (3,4) showed that the impact of a sevenfold expansion of intracellular folate pools in CEM human leukemia cells resulted in a marked loss in growth inhibitory effect by ZD1694 (865-fold) or by another antifolate dependent on polyglutamylation; 5,10-dideazatetrahydrofolate (270-fold). The activity of methotrexate and edatrexate was much less affected, even though their polyglutamylation was impaired. This effect may be consistent with the fact that, even in their monoglutamate forms, methotrexate and edatrexate remain potent inhibitors of dihydrofolate reductase and do not necessarily require polyglutamylation when given in an alternative schedule, such as by continuous administration. More important, our studies (3,4) also identified novel antifolates that display activity, regardless of the intracellular folate status; these include the nonpolyglutamatable antifolates ZD9331 and PT523.

In conclusion, FPGS gene transfer may be a double-edged sword for folate-based chemotherapy. For some types of antifolates, in particular, those that are highly dependent on polyglutamylation for their cytotoxic activity, the concomitant expansion of natural reduced folate cofactor pools may prevent optimal polyglutamylation and thereby confer resistance. For other antifolates—e.g., methotrexate and edatrexate—these possible adverse effects are of lesser importance, so that FPGS gene transfer can contribute to enhanced cytotoxicity (1).

REFERENCES

1 Aghi M, Kramm CM, Breakefield XO. Folylpolyglutamyl synthetase gene transfer and glioma antifolate sensitivity in culture and in vivo. J Natl Cancer Inst 1999;91:1233-41.[Abstract/Free Full Text]

2 Sirotnak FM. Enhancing cytotoxic sensitivity of tumor cells to antifolates: another opportunity for gene therapy? [editorial]. J Natl Cancer Inst 1999;91:1178-9.[Free Full Text]

3 Jansen G, Barr HM, Kathmann I, Bunni MA, Priest DG, Noordhuis P, et al. Multiple mechanisms of resistance to polyglutamatable and lipophilic antifolates in mammalian cells: role of increased folylpolyglutamylation, expanded folate pools, and intralysosomal drug sequestration. Mol Pharmacol 1999;55:761-9.[Abstract/Free Full Text]

4 Jansen G, Mauritz R, Drori S, Sprecher H, Kathmann I, Bunni MA, et al. A structurally altered human reduced folate carrier with increased folic acid transport mediates a novel mechanism of antifolate resistance. J Biol Chem 1998;273:30189-98.[Abstract/Free Full Text]

5 Tse A, Moran RG. Cellular folates prevent polyglutamylation of 5,10-dideazatetrahydrofolate. J Biol Chem 1998;273:25944-52.[Abstract/Free Full Text]

6 Peters GJ, Smitskamp-Wilms E, Smid K, Pinedo HM, Jansen G. Determinants of activity of the antifolate thymidylate synthase inhibitors Tomudex (ZD1694) and GW1843U89 against mono- and multilayered colon cancer cell lines under folate-restricted conditions. Cancer Res 1999;59:5529-35.[Abstract/Free Full Text]

7 O'Dwyer PJ, Nelson K, Thornton DE. Overview of phase II trials of MTA in solid tumors. Sem Oncol 1999;26:99-104.



             
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