Sulfamethoxazole-d4 is intended for use as an internal standard for the quantification of sulfamethoxazole by GC- or LC-MS. Sulfamethoxazole is a sulfonamide antibiotic.1 It inhibits growth of E. coli (MIC = 10 μg/ml) and clinical isolates of methicillin-resistant S. aureus (MRSA; MICs = 25-50 μg/ml).2,3 Sulfamethoxazole, in combination with trimethoprim at a ratio of 20:1, inhibits growth of MRSA in vivo in mice (MIC = 0.8 μg/ml; ED50s = 6.4 and 9.6 mg/kg for two MRSA strains).3 In a mouse model of urinary tract infection with E. coli, a combination of sulfamethoxazole and trimethoprim decreases recurrent infection when administered for 10 days.4 Sulfamethoxazole acts by inhibiting dihydropteroate synthase (DHPS), which converts pteridine and 4-aminobenzoic acid to dihydropteroate, an intermediate in folate biosynthesis. It inhibits recombinant P. carinii DHPS (IC50 = 23 nM; Ki = 7.5 nM) and folate biosynthesis in situ by 48.6%. Formulations containing sulfamethoxazole and trimethoprim have been used to treat bronchitis, prostatitis, and urinary tract infections among other infectious conditions.

|1. Hong, Y.-L., Hossler, P.A., Calhoun, D.H., et al. Inhibition of recombinant Pneumocystis carinii dihydropteroate synthetase by sulfa drugs. Antimicrob. Agents Chemother. 39(8), 1756-1763 (1995).|2. Greenwood, D., and O'Grady, F. Activity and interaction of trimethoprim and sulphamethoxazole against Escherichia coli. J. Clin. Pathol. 29(2), 162-166 (1976).|3. Elwell, L.P., Wilson, H.R., Knick, V.B., et al. In vitro and in vivo efficacy of the combination trimethoprim-sulfamethoxazole against clinical isolates of methicillin-resistant Staphylococcus aureus. Antimicrob. Agents Chemother. 29(6), 1092-1094 (1986).|4. Schilling, J.D., Lorenz, R.G., and Hultgren, S.J. Effect of trimethoprim-sulfamethoxazole on recurrent bacteriuria and bacterial persistence in mice infected with uropathogenic Escherichia coli. Infect. Immun. 70(12), 7042-7049 (2002).