The time-dependent (5 min-72 h) localization of 3 radiolabeled anti-
melanoma monoclonal antibodies (MAbs 436, IND1, and 9.2.27) was studied in paired label experiments in small (4-12 mg) s.c. human
melanoma xenografts (SK-MEL-2 and M21) in athymic nude mice. MAb 436 recognizes a Mr 125,000 cell surface
melanoma-associated
glycoprotein antigen (125 kDa-MAA); MAbs IND1 and 9.2.27 recognize a
high molecular weight melanoma-associated antigen, but with equilibrium association constants differing by 2 orders of magnitude (10(8)-10(10) M-1). The two
tumors were found to differ in their
antigen expression levels and in both interstitial and vascular volumes. Accumulation of MAbs in both
tumors was determined primarily by
antigen expression levels and also by physiological factors such as vascular permeability and vascular volume; at the dose administered (20 micrograms/mouse), differences in MAb affinity among specific MAbs had minimal effect on accumulation. Quantitative flow cytometry measurements showed that
antigen expression in vivo differed from that of cultured tumor cells. In vivo, expression of the Mr 125,000 MAA decreased by
a factor of about 2.5 in both
tumors. In contrast, the in vivo expression of the high molecular weight MAA decreased in M21
tumors but increased by 2.0-3.5-fold in SK-MEL-2
tumors. Data were analyzed using a three-compartment pharmacokinetic model (C. Sung et al.,
Cancer Res., 52:377-384, 1992) to provide plasma-to-tissue transport constants (k), the interstitial fluid flow rate (L), and estimates of the in vivo interstitial MAb binding site concentration (B0). For all MAbs, the plasma-to-tissue transport constants were consistently greater for M21
tumors (0.44-0.85 microliter/min/g) than for SK-MEL-2
tumors (0.28-0.66 microliter/min/g), and values of k for both
tumors were approximately 1 order of magnitude greater than those for skeletal muscle (0.06-0.08 microliter/min/g). The model-estimated binding site concentration of
melanoma-specific
antibodies was 15-70 times lower than that predicted by experimental measurements of
tumor antigen concentrations. Factors that may contribute to this discrepancy include inaccessibility of
tumor cell binding sites to MAb and MAb catabolism. In summary, these results indicate that, for the MAb dose used in this study, variables pertaining to the
tumor target (
i.e., antigen expression levels, vascular volume, and vascular permeability) are the most important for determining MAb accumulation in
tumors.