to 16S rRNA and the RNA-binding S12 protein of the 30S subunit of prokaryotic ribosome and inhibits protein synthesis by changing the ribosome's shape so that it cannot read the mRNA codons correctly. It also interferes with the region that interacts with the wobble base of the tRNA anticodon. It works in a concentration-dependent manner, and has better action in an alkaline environment. At normal doses, amikacin-sensitive bacteria respond within 24–48 hours. Resistance Amikacin evades attacks by all antibiotic-inactivating enzymes that are responsible for antibiotic resistance in bacteria, except for aminoacetyltransferase and nucleotidyltransferase. This is accomplished by the L-hydroxyaminobuteroyl amide (L-HABA) moiety attached to N-1 (compare to kanamycin, which simply has a hydrogen), which blocks the access and decreases the affinity of aminoglycoside-inactivating enzymes. Amikacin ends up with only one site where these enzymes can attack, while gentamicin and tobramycin have six. Bacteria that are resistant to streptomycin and capreomycin are still susceptible to amikacin; bacteria that are resistant to kanamycin have varying susceptibility to amikacin. Resistance to amikacin also confers resistance to kanamycin and capreomycin. Resistance to amikacin and kanamycin in Mycobacterium, the causative agent of tuberculosis, is due to a mutation in the rrs gene, which codes for the 16S rRNA. Mutations such as these reduce the binding affinity of amikacin to the bacteria's ribosome. Variations of aminoglycoside acetyltransferase (AAC) and aminoglycoside adenylyltransferase (AAD) also confer resistance: resistance in Pseudomonas aeruginosa is caused by AAC(6')-IV, which also confers resistance to kanamycin, gentamicin, and tobramycin, and resistance in Staphylococcus aureus and S. epidermidis is caused by AAD(4',4), which also confers resistance to kanamycin, tobramycin, and apramycin. Some strains of S. aureus can also inactivate amikacin by phosphorylating it. Pharmacokinetics Amikacin is not absorbed orally and thus must be administered parenterally. It reaches peak serum concentrations in 0.5–2 hours when administered intramuscularly. Less than 11% of the amikacin actually binds to plasma proteins. It is distributed into the heart, gallbladder, lungs, and bones, as well as in bile, sputum, interstitial fluid, pleural fluid, and synovial fluids. It is usually found at low concentrations in the cerebrospinal fluid, except when administered intraventricularly. In infants, amikacin is normally found at 10–20% of plasma levels in the spinal fluid, but the amount reaches 50% in cases of meningitis. It does not easily cross the blood–brain barrier or enter ocular tissue. While the half-life of amikacin is normally two hours, it is 50 hours