Background: The metastasis of prostate cancer to bone is associated with a substantial increase in bone matrix turnover. Matrix metalloproteinases (MMPs) play roles in both normal bone remodeling and invasion and metastasis of prostate cancer. This study was designed to determine the role of MMP activity in prostate cancer that has metastasized to bone.
Methods: Single human fetal bone fragments were implanted subcutaneously in immunodeficient mice. Four weeks later, PC3 human prostate cancer cells were injected directly into some of the implants, and daily treatment was begun with batimastat (a broad-spectrum MMP inhibitor). There were six mice (i.e., six implants) in each of four experimental arms: bone alone with and without batimastat and bone injected with PC3 cells with and without batimastat. Bone implants were harvested after 14 days of treatment and analyzed for MMP expression, bone histomorphometry, osteoclast counts, blood vessel density, and tumor cell proliferation and apoptosis. Complementary data were obtained from bone biopsy samples from patients and a bone organ coculture system. All statistical tests were two-sided.
Results: MMPs were detected in tumor and stromal cells of clinical specimens and experimental bone implants. In vivo, MMP inhibition reduced the number of osteoclasts per millimeter in PC3-injected implants-from 8.2 (95% confidence interval [CI] = 7.9 to 8.5) to 3.0 (95% CI = 2.3 to 3.7) (P =.006). In addition, it prevented degradation of marrow trabeculae within the bone implants (cross-sectional area of implant occupied by mineralized trabeculae: untreated implant = 29.1% [95% CI = 27.1% to 31.1%], PC3-injected implant = 14.0% [95% CI = 10.9% to 17.1%] [P =.005 versus untreated], and batimastat-treated PC3-injected implant = 27.2% [95% CI = 22.4% to 32.0%] [P =.03 versus PC3 injected alone]). MMP inhibition reduced proliferating tumor cells from 20.8% (95% CI = 19.9% to 21.7%) to 7.4% (95% CI = 5.2% to 9.6%) (P =.006), without affecting angiogenesis or apoptosis. In vitro, MMP inhibition had no toxic effect on PC3 cells but prevented calcium release from bone fragments cocultured with PC3 cells.
Conclusions: MMP activity appears to play an important role in bone matrix turnover when prostate cancer cells are present in bone. Bone matrix turnover and metastatic tumor growth appear to be involved in a mutually supportive cycle that is disrupted by MMP inhibition.