Linomide Inhibits Angiogenesis, Growth, Metastasis, and Macrophage Infiltration within Rat Prostatic Cancers

Linomide, a quinoline-3-carboxamide, has the ability to inhibit the growth of prostatic cancer in vivo but not in vitro (T. Ichikawa et al. , Cancer Res., 52: 3022–3028, 1992). The reason for this discrepancy is that linomide inhibits tumor growth not directly but indirectly in vivo via its ability to inhibit the angiogenic response induced within the growing prostatic cancer (J. Vukanovic, et al. , Cancer Res., 53: 1833–1837, 1993). Tumor associated macrophages can stimulate angiogenesis via their ability to secrete various cytokines, particularly tumor necrosis factor α (TNF-α). Treatment of rats with linomide decreases significantly ( P < 0.05), by more than 50%, the number of tumor associated macrophages within both locally invasive ( i.e. , from 20–40 to 10 macrophages/high power field) and highly metastatic primary prostatic cancers ( i.e. , from 60–70 to 15–37 macrophages/high power field). Monocytes/macrophages isolated from linomide treated rats had a decreased ability to secrete TNF-α when challenged in vitro with the bacterial endotoxin, lipopolysaccharide [ i.e. , 702 ± 76 (SEM) ng of TNF-α/105 monocytes/macrophages from control versus 401 ± 2 ng of TNF-α/105 monocytes/macrophages from linomide treated rats]. In addition, when rats were treated with linomide and than challenged with lipopolysaccharide in vivo , the resulting elevation in serum TNF-α was inhibited by ∼50% ( i.e. , 4.56 ± 1.8 ng/ml of TNF-α in control versus 2.9–2.2 ng/ml depending upon the dose of linomide). The ability of linomide to decrease monocyte/macrophage secretion of TNF-α is not immediate, however, since the secretion of TNF-α induced by lipopolysaccharide challenge of monocytes/macrophages isolated from untreated animals is not decreased by acute ( i.e. , <4 h) linomide treatment in vitro . These results demonstrate that the ability of linomide to inhibit the secretion of TNF-α by monocytes/macrophages requires either additional time or host factors. To test if natural killer (NK) cells might be one of the additional host factors required for the in vivo abilities of linomide, prostatic cancer bearing rats were treated with appropriate antiserum to deplete NK cells and then tested for their response to linomide treatment. These studies demonstrated that the antitumor, antimetastatic, and antimacrophage effects of linomide were unaffected by NK cell depletion. Thus, the antiangiogenic effect of linomide is NK independent and is associated with inhibition of tumor associated macrophage number and their ability to secrete TNF-α.