Biodistribution and tumour localization of a methotrexate-monoclonal-antibody 791T/36 conjugate in nude mice with human tumour xenografts.

The blood kinetics and tumour localization of a conjugate of methotrexate (MTX) and MAb 791T/36 were examined in nude mice with human tumour xenografts. The antibody moiety of the conjugate was detected by labelling with 125I and the drug moiety was assayed using a radioimmunoassay for methotrexate. After radioiodination, the drug moiety was co-precipitable with the radiolabel when TCA or rabbit anti-mouse IgG antiserum was used. Following i.v. injection, serum kinetics of both the antibody and the drug moieties of the conjugate were essentially similar, and the integrity of serum-borne conjugate was confirmed by the co-precipitation of radiolabel and drug. The radiolabelled antibody moiety of the conjugate localized in tumour xenografts, with 5-7% of the injected dose being present per gram of tissue within 6 hr of injection, and the levels were maintained for up to 4 days. Analysis of tumour levels of the MTX moiety showed a progressive uptake over the 4-day observation period with up to 4% of the injected dose being present per gram of tumour when the experiment was terminated. Parallel studies with free MTX showed rapid clearance from the blood and a maximum of 0.35% of the dose/g of tumour 30 min after injection. Control immunoglobulin conjugated to MTX did not show tumour localization of either the antibody or the drug moieties. These studies confirm that in vivo MTX remains bound to antibody in this type of drug antibody conjugate and demonstrate site-specific targeting of this therapeutic agent.