Photodynamic therapy (PDT) may be improved by enhancing the delivery of photosensitizers (PS) to selected lesions using targeted macromolecular conjugates. Recently it has become accepted that tumor-associated macrophages (TAMs) assist the tumor to grow and spread by several distinct mechanisms (paracrine growth factors, increased angiogenesis, and matrix-degrading enzymes) and they have been proposed to be a valid target for cancer therapy. This revised proposal investigates a new approach to killing TAMs by targeted delivery of modified albumin-chlorin (e6) conjugates that are recognized by the scavenger receptors present on TAMs, together with tumor-confined illumination. We have shown that this approach allows very specific photodestruction of mouse macrophages in vitro and leads to substantial inhibition of tumor growth in vivo in both macrophage and non-macrophage tumors. This proposal will test the hypothesis that the combination of macrophage selectivity and directed illumination will kill TAMs without harming other distant macrophage populations, and hence produce beneficial tumor responses including growth delay, decreased angiogenesis and metastasis, increased survival, and development of tumor immunity. The interaction of the conjugates with macrophages is likely to depend on their cellular activation state and this will be investigated with gene expression arrays and quantitation of scavenger-receptor expression by RT-PCR. J774 cells form highly aggressive and metastatic s.c. tumors in BALB/c mice and the biodistribution and PDT response of these conjugates will be compared to free PS. Immunohistochemistry will allow microvessel density, macrophage content, and proliferative index to be determined in frozen sections from treated tumors. The PDT responses of a pair of s.c. mouse tumors differing in macrophage content and immunogenicity (EMT-6 and RIF- 1) will be determined with quantitative comparison of targeted and non-targeted PDT at roughly equal effective doses. The enhancement of PDT by an adjuvant (OK432) designed to increase the degree of tumor infiltration by TAMs and to increase their activation state will be explored. It is proposed that a PDT response which is inflammatory will encourage the induction of a specific anti-tumor immune response, which will be explored by rechallenging cured animals with the same and unrelated cell lines, and measurement of effector cell functions (cytotoxic T lymphocyte, natural killer cell and macrophage) from spleens and draining lymph nodes.
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