Effect of metrizamide, a nonionic radiographic contrast agent, on human serum complement. Comparison with ionic contrast media.

The nonionic radiographic contrast material (RCM) metrizamide causes consumption of total complement activity in normal human serum (NHS) in vitro in the absence and to a lesser extent also in the presence of EDTA. The depression of titers of total complement is related to an inactivating effect of metrizamide on component C2. Furthermore, metrizamide induces activation of the alternative pathway as evidenced by the appearance of C3 and factor B cleavage products in NHS, dependent on the presence of divalent cations. Alternative pathway activation is probably mediated by an antagonizing effect of metrizamide on the inactivation of C3b. Unlike ionic RCM, the nonionic substance metrizamide does not lead to cleavage of the internal thiolester bond present in native C3 and C4, at concentrations that produce potent consumption of C3 activity in NHS.