Spectinomycin dihydrochloride pentahydrate is a broad-spectrum aminocyclitol antibiotic that inhibits the growth of a variety of gram-positive and gram-negative organisms. In Vitro: Spectinomycin selectively inhibits protein synthesis in cells and in extracts of Escherichia coli. When added to an exponentially growing culture, spectinomycin (50 μg/mL) rapidly and reversibly inhibits growth of Escherichia coli. Amino acid incorporation is slowed immediately but RNA synthesis continued. In extracts of Escherichia coli B, spectinomycin inhibits polypeptide synthesis directed either by endogenous messenger RNA or by MS-2 bacteriophage RNA. Maximum inhibition (70 to 80%) is achieved at 1 μg/mL (3 μM). ^[1]. Spectinomycin blocks the translocation of peptidyl-tRNAs from the A-site to the P-site by inhibiting the binding of elongation factor G to the ribosome. Spectinomycin interacts specifically with the residues G1064 and 01192 in 16S rRNA and potentially change this molecule into an inactive conformation^[2]. Spectinomycin acts as a mixed noncompetitive inhibitor for the td intron RNA with a K_i of 7.2 mM. The splicing inhibition by spectinomycin is dependent on pH changes and Mg²⁺ concentration, indicating electrostatic interactions with the intron RNA^[3]. In Vivo: Renal excretion is a major elimination pathway for spectinomycin. Following IV administration, approximately 55% of the drug is excreted into the urine in unchanged form. After IV administration of 10 mg/kg spectinomycin shows a peak plasma concentration of 37.8 μg/mL and a systemic exposure (area-under the curve AUC_0-∞) of 15.7 μg/mL. Following single dose intramuscular administration, the overall elimination half-life of spectinomycin is 1.2 h in cattle, 1.0 h in sheep, 1.0 h in pigs, 1.65 h in chicken and 1.85 h in humans^[4].