Gentamicin

Gentamicin is not recommended in pregnancy unless the benefits outweigh the risks for the mother. Gentamicin can cross the placenta and several reports of irreversible bilateral congenital deafness in children have been seen. Intramuscular injection of gentamicin in mothers can cause muscle weakness in the newborn.[13]

The safety and efficacy for gentamicin in nursing mothers has not been established. Detectable gentamicin levels are found in human breast milk and in nursing babies.[13]

In the elderly, renal function should be assessed before beginning therapy as well as during treatment due to a decline in glomerular filtration rate. Gentamicin levels in the body can remain higher for a longer period of time in this population. Gentamicin should be used cautiously in persons with renal, auditory, vestibular, or neuromuscular dysfunction.[12]

Gentamicin may not be appropriate to use in children, including babies. Studies have shown higher serum levels and a longer half-life in this population.[17] Kidney function should be checked periodically during therapy. Long-term effects of treatment can include hearing loss and balance problems. Hypocalcemia, hypokalemia, and muscle weakness have been reported when used by injection.[12]

Kidney damage is a problem in 10–25% of people who receive aminoglycosides, and gentamicin is one of the most nephrotoxic drugs of this class.[18] Oftentimes, acute nephrotoxicity is reversible, but it may be fatal.[12] The risk of nephrotoxicity can be affected by the dose, frequency, duration of therapy, and concurrent use of certain medications, such as NSAIDs, diuretics, cisplatin, ciclosporin, cephalosporins, amphotericin, iodide contrast media, and vancomycin.[18]

Factors that increase risk of nephrotoxicity include:[18]

• Increased age
• Reduced renal function
• Pregnancy
• Hypothyroidism
• Hepatic dysfunction
• Volume depletion
• Metabolic acidosis
• Sodium depletion

Kidney dysfunction is monitored by measuring creatinine in the blood, electrolyte levels, urine output, presence of protein in the urine, and concentrations of other chemicals, such as urea, in the blood.[18]

About 11% of the population who receives aminoglycosides experience damage to their inner ear.[19] The common symptoms of inner ear damage include tinnitus, hearing loss, vertigo, trouble with coordination, and dizziness.[20] Chronic use of gentamicin can affect two areas of the ears. First, damage of the inner ear hair cells can result in irreversible hearing loss. Second, damage to the inner ear vestibular apparatus can lead to balance problems.[20] To reduce the risk of ototoxicity during treatment, it is recommended to stay hydrated.[12]

Factors that increase the risk of inner ear damage include:[12][13]

• Increased age
• High blood uric acid levels
• Kidney dysfunction
• Liver dysfunction
• Higher doses
• Long courses of therapy
• Also taking strong diuretics (e.g., furosemide)

Gentamicin is a bactericidal antibiotic that works by binding the 30S subunit of the bacterial ribosome, negatively impacting protein synthesis. The primary mechanism of action is generally accepted to work through ablating the ability of the ribosome to discriminate on proper transfer RNA and messenger RNA interactions.[21] Typically, if an incorrect tRNA pairs with an mRNA codon at the aminoacyl site of the ribosome, adenosines 1492 and 1493 are excluded from the interaction and retract, signaling the ribosome to reject the aminoacylated tRNA::Elongation Factor Thermo-Unstable complex.[22] However, when gentamicin binds at helix 44 of the 16S rRNA, it forces the adenosines to maintain the position they take when there is a correct, or cognate, match between aa-tRNA and mRNA.[23] This leads to the acceptance of incorrect aa-tRNAs, causing the ribosome to synthesize proteins with wrong amino acids placed throughout (roughly every 1 in 500).[24] The non-functional, mistranslated proteins misfold and aggregate, eventually leading to death of the bacterium. A secondary mechanism has been proposed based on crystal structures of gentamicin in a secondary binding site at helix 69 of the 23S rRNA, which interacts with helix 44 and proteins that recognize stop codons. At this secondary site, gentamicin is believed to preclude interactions of the ribosome with ribosome recycling factors, causing the two subunits of the ribosome to stay complexed even after translation completes. This creates a pool of inactive ribosomes that can no longer re-initiate and translate new proteins.[25]

Since gentamicin is derived from the species Micromonospora, the backbone for this antibiotic is the aminocyclitol 2-deoxystreptamine.[26][27] This six carbon ring is substituted at the carbon positions 4 and 6 by the amino sugar molecules cyclic purpurosamine and garosamine, respectively.[28][26] The gentamicin complex, is differentiated into five major components (C₁, C_1a, C₂, C_2a, C_2b) and multiple minor components by substitution at the 6' carbon of the purpurosamine unit indicated in the image to the right by R¹ and R².[28][26][29][30] The R¹ and R² can have the follow substitutions for some of the species in the gentamicin complex.[28][31][27]

Gentamicin is composed of a number of related gentamicin components and fractions which have varying degrees of antimicrobial potency.[32] The main components of gentamicin include members of the gentamicin C complex: gentamicin C₁, gentamicin C_1a, and gentamicin C₂ which compose approximately 80% of gentamicin and have been found to have the highest antibacterial activity. Gentamicin A, B, X, and a few others make up the remaining 20% of gentamicin and have lower antibiotic activity than the gentamicin C complex.[30] The exact composition of a given sample or lot of gentamicin is not well defined, and the level of gentamicin C components or other components in gentamicin may differ from lot-to-lot depending on the gentamicin manufacturer or manufacturing process. Because of this lot-to-lot variability, it can be difficult to study various properties of gentamicin including pharmacokinetics and microorganism susceptibility if there is an unknown combination of chemically related but different compounds.[33]

The complete biosynthesis of gentamicin is not entirely elucidated. The genes controlling the biosynthesis of gentamicin are of particular interest due to the difficulty in obtaining the antibiotic after production.[30][29][31][34][35] Since gentamicin is collected at the cell surface and the cell surface must be perforated some how to obtain the antibiotic.[30][29][31][34][35] Many propose the amount of gentamicin collected after production could increase if the genes are identified and re-directed to secrete the antibiotic instead of collecting gentamicin at the cell surface.[30][29][31][34][35] Literature also agrees with the gentamicin biosynthesis pathway starting with D-Glucose-6-phosphate being dephopsphorylated, transaminated, dehydrogenated and finally glycosylated with D-glucosamine to generate paromamine inside Micromonospora echinospora.[28] The addition of D-xylose leads to the first intermediate of the gentamicin C complex pathway, gentamicin A2.[28][36] Gentamicin A2 is C-methylated and epimerized into gentamicin X₂, the first branch point of this biosynthesis pathway[36]

When X₂ is acted on by the cobalamin-dependent radical S-adenosyl-L-methionine enzyme GenK, the carbon position 6' is methylated to form the pharmacologically active intermediate G418[37][36][28][38] G418 then undergoes dehydrogenation and amination at the C6' position by the dehydrogenase gene, GenQ, to generate the pharmacologically active JI-20B, although another intermediate, 6'-dehydro-6'oxo-G418 (6'DOG) is proposed to be in-between this step and for which the gene GenB1 is proposed as the aminating gene.[28][39] JI-20B is dehydroxylated and epimerized to first component of the gentamicin C complex, gentamicin C2a which then undergoes an epimerization by GenB2 and then a N-methylation by an unconfirmed gene to form the final product in this branch point, gentamicin C1.[36][39][28][40]

When X₂ bypasses GenK and is directly dehydrogenated and aminated by the GenQ enzyme, the other pharmacologically relevant intermediate JI-20A is formed.[28][39] Although, there has been identification of an intermediate for this step, 6'-dehydro-6'-oxo-gentamicin X2 (6'-DOX), for which the enzyme GenB1 is purposed as the aminating enzyme.[39] JI-20A is then dehydroxylated into the first component of the gentamicin C complex for this branch, gentamicin C1a via a catalytic reaction with GenB4.[40] C1a then undergoes an N-methylation by an unconfirmed enzyme to form the final component, gentamicin C2b.[39][36][28][40]

Gentamicin is only synthesized via submerged fermentation and inorganic sources of nutrients have been found to reduce production.[28] Traditional fermentation used yeast beef broth,[29] but there has been research into optimizing the growth medium for producing gentamicin C complex due to the C complex currently being the only pharmaceutically relevant component.[28] The main components of the growth medium are carbon sources, mainly sugars, but several studies found increased gentamicin production by adding vegetable and fish oils and decreased gentamicin production with the addition of glucose, xylose and several carboxylic acids.[28] Tryptone and various forms of yeast and yeast derivatives are traditionally used as the nitrogen source in the growth medium, but several amino acids, soybean meal, corn steep liquor, ammonium sulfate, and ammonium chloride have proven to be beneficial additives.[28][31] Phosphate ions, metal ions (cobalt and a few others at low concentration), various vitamins (mostly B vitamins), purine and pyrimidine bases are also supplemented into the growth medium to increase gentamicin production, but the margin of increase is dependent on the species of Micromonospora and the other components in the growth medium.[28][34] With all of these aforementioned additives, pH and aeration are key determining factors for the amount of gentamicin produced.[28][31] A range of pH from 6.8 to 7.5 is used for gentamicin biosynthesis and the aeration is determined by independent experimentation reliant on type of growth medium and species of Micromonospora.[28][31]