Cancer Immunol. Immunother. 9, 245-253 (1980) ancer mmunol_ogyand mmunotherapy © Springer-Verlag 1980 A Mouse Model for Immunotherapy of Osteosarcoma III. Combined Therapy with Corynebacterium parvum and Prostaglandin Synthetase Inhibitors Paul Gatenby 1, Antony Basten 2, and Paul Creswick 2 1 Clinical Immunology, Royal Prince Alfred Hospital, Camperdown, N.S.W. 2050 2 Immunology Unit, Medical School, Sydney University, N.S.W. 2006, Australia Summary. Various combinations of the prostaglan- dins synthetase inhibitors, aspirin or indomethacin, and Corynebacterium parvum were used as adjunctive therapy to surgery in the treatment of the metastasizing Dunn osteosarcoma C3H/HeJ mice. In doses corre- sponding to those that could be tolerated by humans all three agents given singly reduced the number and estimated weight of metastases whether they were administered pre-operatively or post-operatively. When the two modalities of therapy were combined a significant additive anti-tumour effect was observed with both C. parvum and aspirin and C. parvum and indomethacin, but only if treatment was commenced pre-operatively. Similar results were obtained with two other metastasizing turnouts, viz. the B16 melanoma and the Lewis lung carcinoma. In the absence of any evidence for an interaction between the prostaglandin synthetase inhibitors and circulating tumour cells, it was felt that the additive effect could be best explained in terms of interference with prostaglandin-mediated negative feedback of the anti-tumour action of C. parvum. Introduction Prostaglandins (PGs) have recently been shown to exert an influence on tumour growth and spread [22, 33, 36, 38, 39]. However, the mechanisms involved in this interaction are complex since PGs, even of the same series (PGEs) have been found to exert an anti-tumour effect both in vivo [36] and in vitro [35], while subverting anti-tumour responses in other systems [22, 33]. The evidence for a tumour-poten- tiating role of PGs comes largely from studies showing that PG synthetase inhibitors, e.g., indo- methacin or aspirin, can reduce the growth of Reprint requests should be adressed to: P. Gatenby tumours in certain animal systems [22, 26, 33]. The mechanisms by which they do this is unclear, but it is apparent that some tumours can secrete PGs [8, 39]. In addition, activated macrophages [19, 20, 30], T cells [19], and even B cells [19] may produce PGs, and there are ample studies in man and animals impli- cating PGEs as local feedback inhibitors of T-cell activation in vitro and in vivo [reviewed in 19]. The general state of immunodepression that accompanies disseminated malignancy has been shown to be mediated by a PGE-dependent suppressor mecha- nism in some animal [19, 33] and human turnouts [20], but not in others [19]. Three potential sites exist for the anti-turnout actions of PGEs. They may act peripherally on mononuclear phagocytes [37] or natural killer (NK) cells [7, 8] or may exert their inhibitory effect centrally, by reducing the output of mononuclear phagocytes from the myelomonocytic progenitor cells in the bone marrow [24, 32]. Systemic Corynebacterium parvum is thought to exert its non-specific anti-tumour effect by increasing the output of mononuclear phagocytes from the marrow [6, 10, 12, 42] and/or by activating macrophages [3, 17], and NK cells via interferon [1, 2, 40]. These mechanisms are illustrated schematically in Fig. 1, together with the potential sites at which PGEs may interfere with them. The synthesis of PGEs depends on the enzyme prostaglandin synthetase or cycloox- ygenase [28], and therefore any drugs which interfere with the action of this enzyme will lead to a reduction in PGEs. Consequently, an investigation of the combined anti-tumour effect of C. parvum and prostaglandin synthetase inhibitors seems logical on theoretical grounds. The PG synthetase inhibitors aspirin and indo- methacin have in fact been shown to exert an anti-tumour effect alone and to potentiate the immunotherapeutic agents BCG and C. parvum in a model involving flank tumours in C3H mice [26]. The 0340-7004/80/0009/0245/$ 01.80