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Spatial Distribution of Tumor-specific Monoclonal Antibodies in Human Melanoma Xenografts¹

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 [T. R. S.]; Departments of Pathology, Beth Israel Hospital and Harvard Medical School, and the Charles A. Dana Research Institute, Beth Israel Hospital [K. L., J. A. N., H. F. D.], and Surgical Services and Department of Surgery, Massachusetts General Hospital and Harvard Medical School [R. G. T., M. L. Y.], Boston, Massachusetts 02215; and Department of Chemical and Biochemical Engineering and the Center for Advanced Biotechnology and Medicine, Rutgers University, Piscataway, New Jersey, 08854 [M. L. Y.]

The time-dependent (1-72-h) spatial distribution of three biotinylated anti-melanoma monoclonal antibodies (MAbs), a control MAb, and several macromolecular tracers was studied in two small (4–12-mg), well-characterized human melanoma xenografts (SK-MEL-2, M21) growing in the s.c. space of athymic nude mice. The specific MAbs (436, IND1, and 9.2.27) recognize two different melanoma cell surface antigens (M_r 125,000 glycoprotein melanoma-associated antigen and high molecular weight melanoma-associated antigen) and have equilibrium association constants differing by two orders of magnitude (10⁸–10¹⁰ M^-1). SK-MEL-2 tumors were poorly vascularized and were composed of one or several collections of tumor cells with few intratumor blood vessels. In contrast, M21 tumors induced a strong angiogenic response and were organized into multiple small tumor cell nests separated from each other by fine blood vessels. Neither tumor developed extensive connective tissue stroma. In both tumors, hyperpermeable blood vessels were concentrated at the tumor-host interface but some intratumor vessels in M21 tumors were also leaky. Macromolecular tracers extravasated extensively from leaky vessels into tumor stroma but penetrated poorly into tumor parenchyma. All three tumor-specific MAbs stained tumor cell surfaces in a time-dependent fashion such that one-half or more of all tumor cells were stained by 24–48 h. Tumor cell staining was favored by increased density of tumor cell antigens but, at the doses studied, was little affected by differences in affinity among tumor-specific antibodies. The distribution of MAb staining was nonuniform in two respects: (a) peripherally situated tumor cells were more likely to be stained than centrally placed cells, and only in the smallest tumors did MAb reach centrally placed tumor cells; and (b) staining was nonuniform in different parts of the same tumor. The inhomogeneity of tumor cell staining by tumor-specific MAb was attributable to several factors, including: tumor blood vessel number, distribution, perfusion and permeability; distribution of tumor connective tissue stroma; small volume of the parenchymal interstitial space and relatively impaired diffusion of macromolecules in that space (low effective diffusivity of MAb); and interactions between specific MAbs and tumor cells. Of these factors, those associated with the parenchymal compartment apparently were rate limiting, and strategies that enhance parenchymal penetration are likely to improve solid tumor therapy with MAbs.

¹ This work was supported by grants from the Lucille P. Markey Charitable Trust (M. L. Y.), the Whitaker Foundation for Biomedical Engineering (M. L. Y.), and by NIH Research Grants CA-28471 and CA-50453 (H. F. D.).

² Present address: Baxter Health Care Corp., Route 120 and Wilson Road, Mail Code WG1-3S, Round Lake, IL 60073

³ To whom requests for reprints should be addressed, at Department of Pathology, Beth Israel Hospital, 330 Brookline Avenue, Boston, MA 02215.

Received 6/19/91. Accepted 11/ 1/91.

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