Methotrexate (MTX) was conjugated to an immunoglobulin G1 (IgG1) monoclonal antibody specific for human prostatic acid phosphatase (PAP) by the active ester method. The molar ratio of MTX to IgG was 14. MTX-monoclonal antibody conjugate retained substantially the original PAP-binding inhibition activity of the monoclonal antibody. Both MTX-monoclonal antibody conjugate and an identically prepared MTX-normal mouse IgG conjugate preserved 90% of the original dihydrofolate reductase inhibitory activity of MTX. [3H]MTX conjugated to monoclonal anti-PAP antibody was significantly accumulated more in PAP-producing human prostate tumor LNCaP cells than its normal mouse IgG counterpart. No statistical difference was found between the uptake of [3H]MTX conjugated to monoclonal antibody and that of [3H]MTX conjugated to normal mouse IgG by control PAP nonproducing thyroid tumor cells (TT). The antitumor effect of the conjugate was evaluated in vitro by its inhibition on deoxy[6-3H]uridine incorporation into LNCaP cells. The inhibition by MTX-monoclonal antibody conjugate was significantly higher than that by MTX-normal mouse IgG conjugate at 8 µg of drug per ml, although it was significantly less than that by free MTX. However, an in vivo tumor and tissue distribution study of [3H]MTX and its conjugates revealed that, 5 days after i.v. administration, [3H]MTX conjugated to monoclonal antibody was preferentially accumulated in LNCaP prostate tumor. Tumor:blood ratios for [3H]MTX, [3H]MTX-monoclonal antibody conjugate, and [3H]MTX-normal mouse IgG conjugate were 1.47, 5.06, and 1.26, respectively. Preliminary results obtained from a pilot study with a small number of animals demonstrated that multiply injected MTX-monoclonal antibody conjugate retarded the growth of xenografted prostate tumor (LNCaP) as compared with the control groups, including free MTX which showed a shorter period of therapeutic effectiveness. This study suggests that MTX conjugated to monoclonal anti-PAP antibody could be a potential reagent for experimental immunochemotherapy of prostate tumor, should the initial in vivo data be extended and confirmed.

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Supported in part by Research Grants CH-269, awarded by the American Cancer Society, and CA-34536 and CA-15437, awarded by the National Cancer Institute.

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