Iclaprim is an investigational broad-spectrum diaminopyrimidine antibiotic in development for the treatment of acute bacterial skin and skin structure infections (ABSSSIs). Iclaprim acts on bacterial cells by competitively inhibiting dihydrofolate reductase (DHFR), a key enzyme in the folate cycle; the same mode of inhibition is exerted by trimethoprim. Iclaprim resistance is mainly determined by point mutations in the dfr gene as studied in S. aureus and S. pneumoniae. Surveillance studies demonstrate that the spectrum of activity of iclaprim includes many organisms indicated in cSSSI including S. aureus and S. pyogenes. Iclaprim is bactericidal in vitro, generally at concentrations equal to the MIC that are maintained in human plasma for several hours after a therapeutic dose. Bactericidal activity is primarily time-dependent and concentration independent. Due to its structural similarity with trimethoprim, iclaprim is synergistic with sulfonamides against a broad spectrum of bacterial species. The antimicrobial mechanism of action of iclaprim is mediated by competitive inhibition of bacterial DHFR, the same mode of inhibition exerted by TMP. The activity of iclaprim against TMP-R mutants of S. aureus and S. pneumoniae is attributable to additional hydrophobic interaction between iclaprim and the enzyme. The same mechanism of action of iclaprim, competitive inhibition with the natural substrate DHF, is seen against both TMP-S and -R enzymes. Iclaprim is well suited for use as a first-line empiric monotherapy in patients with ABSSSI who are comorbid with renal impairment for the following reasons. n July 2015, the U.S. Food and Drug Administration, or FDA, designated the IV formulation of iclaprim as a Qualified Infectious Disease Product (QIDP) for ABSSSI and HABP. QIDP status grants iclaprim regulatory Fast Track designation, Priority Review and, if approved, a five-year extension to the statutory market exclusivity period in the United States, resulting in 10 years of market exclusivity from the date of approval.