Objective. Collagen-induced arthritis (CIA) in the rhesus monkey is a nonhuman primate model of rheumatoid arthritis (RA). The close phylogenetic relationship between humans and the rhesus monkey makes this model useful for the preclinical safety and efficacy testing of new therapies that are inactive in animals more distinctly related to humans. In this study, we tested the therapeutic potential of a novel, small molecular weight antagonist of CCR5, SCH-X, in this model.Methods. CIA was induced in 10 rhesus monkeys. The animals were allocated to receive SCH-X or saline as the control (n ؍ 5 in each group). Treatment was initiated on the day of CIA induction and continued for 45 days. Monkeys were monitored before and 63 days after CIA induction for macroscopic signs of clinical arthritis, such as soft-tissue swelling and body weight. Furthermore, markers of inflammation and joint degradation were monitored to follow the disease course.Results. Only 2 of 5 animals in the SCH-Xtreated group displayed prominent soft-tissue swelling, compared with all 5 saline-treated monkeys. In addition to the suppression of joint inflammation, treatment with SCH-X resulted in a reduction in joint destruction, as demonstrated by lower rates of urinary excretion of collagen crosslinks, with confirmation by histology. Whereas in all saline-treated monkeys, marked erosion of joint cartilage was observed, this was absent in 4 of the 5 SCH-X-treated monkeys.Conclusion. The systemic effects of treatment with SCH-X were a suppressed acute-phase reaction (reduction in C-reactive protein level) in the 3 treated monkeys with CIA that remained asymptomatic, and an altered antibody response toward type II collagen. The results suggest that the CCR5 antagonist SCH-X might have a strong clinical potential for treatment during periods of active inflammation, as seen in RA.Chemokines are involved in the trafficking, localization, and differentiation of leukocytes, as well as in effector functions (1). Certain chemokines are produced only during conditions of inflammation and are held responsible for the specific attraction of cells toward inflammatory sites. By the selective expression of chemokine receptors on specific T cell subsets, chemokines are believed to contribute to specific features of the inflammation process and the resulting tissue destruction by the attraction of cells with specific functional properties. This is illustrated by the preferential expression of CCR5 and CXCR3 on Th1 cells (2,3) and of CCR4 and CCR3 on Th2 cells (4). In addition to the selective expression on in vitro-generated Th1 cell lines, target organs in Th1-associated autoimmune diseases, such as the central nervous system in multiple sclerosis (MS), the thyroid gland in Graves' disease, and synovial joints in rheumatoid arthritis (RA), are enriched with T cells expressing CCR5 and/or CXCR3 (5-12). Moreover, increased levels of CCR5 and CXCR3 ligands have been detected in areas of inflammation and/or the Drs. Tagat, Steensma, and McCombie hold a patent on SCH...