Aminoglycosides

3.1 Aminoglycosides

The aminoglycosides each contain one or more amino sugars, for instance: neosamine or glucosamine, bridged by glycoside linkages to a basic, either amino or guanidino, six-membered carbon ring, such as: streptamine or streptidine as given below:

Aminoglycosides occupy a coveted status in the domain of antibiotics exclusively for the control, management and treatment of infections caused by Gram-negative bacilli. However, the overall treatment of most nosocomial Gram negative basillary infections with the aid of third-generation cephalosporins, carbapenems and new fluoroquinolones have made the aminoglycosides more or less as the alternative drugs unless and until resistant strains are suspected invariably amongst the immuno suppressed patients.

It is worthwhile to mention here that the major spectrum of activity of the aminoglycosides essentially comprise of aerobic Gram-negative bacilli and Staphylococcus aureus.

A few important members of the aminoglycoside antibiotics are, namely: amikacin, gentamycin, paramycin, kanamycin, netilmicin, streptomycin and tobramycin which shall be discussed separately as under:

3.1.1 Amikacin

Synonyms Lukadin.

Biological Source It is a semisynthetic aminoglycoside antibiotic derived from Kanamyci A.

Chemical Structure

1-N-[L (-)-4-Amino-2-hydroxybutyryl] kanamycin A; (C₂₂H₄₃N₅O₁₃).

The presence of the 4-amino-2-hydroxybutyryl moiety protects the antibiotic against the enzymic deactivation at many locations, while the activity of the parent molecule is still maintained.

Preparation Amikacin is obtained by acylation of the C-1 amino function of the 2-deoxystreptamine group of kanamycin with L-(-)-4-amino-2-hydroxybutyric acid.

Characteristic Features

1. It is obtained as white crystalline powder from a mixture of methanol-isopropanol having m_p 203-204°C (sesquihydrate).

2. Its specific optical rotation [α]²³_D + 99° (C = 1.0 in water).

Identification Tests

Amikacin Sulphate (C₂₂H₄₃N₅O₁₃.2H₂SO₄) (Amikin; Amiklin; Biklin; Amikavet; Fabianol; Kaminax; Mikavir; Novamin; Pierami): It is obtained as an amorphous form which gets decomposed at 220-230°C. Its specific optical rotation [α]_D²² + 74.75° (water).

Uses

1. Amikacin is observed to be fairly stable to most of the aminoglycoside inactivating enzymes, and is, therefore, considered to be valuable for the treatment of serious infections usually caused by Gram –ve bacteria that are resistant to gentamycin or tobramycin.

2. It is mostly employed in a wide range of infections, such as: septicemia, serious infections due to burns, urinary tract, respiratory tract and various soft tissues, meningitis, peritonitis, osteomyelitis, omphalitis in neonates, and other serious surgical infections.

3.1.2 Gentamicin

Synonym Gentamycin.

Biological Sources It is an antibiotic complex produced by the fermentation of Micromonospora purpurea and M. echinospora; and a number of variants thereof.

Chemical Structure

Preparation Gentamycin is normally recovered from a fermentation broth produced when submerged cultures of two subspecies of Micromonospora purpurea are grown in the yeast extractcerelose medium.

Characteristic Features

1. It is a white amorphous powder having mp 102-108°C

2. It has specific optical rotation [α]_D²⁵ + 146°.

3. It is found to be freely soluble in water, pyridine, DMF, in acidic media with salt formation; moderately soluble in methanol, ethanol, acetone; and almost insoluble in benzene and halogenated hydrocarbons. Characteristic features of some of its congeners are as follows:

Identification Tests

1. Gentamicin Complex Sulphate: [Synonyms Alcomicin; Bristagen; Cidomycin; Duragentum; Garamycin; Garasol; Genoptic; Gentacin; Gentak; Gentalline; Gentalyn; Gentibioptal; Genticin; Gentocin; Gentogram; Gent-Ophtal; Gentrasul; Lugacin; Nichogencin; Ophtagram; Pangram; Refobacin; Septopal; Sulmycin; and U-Gencin.]

It is obtained as a white, hygroscopic powder having mp 218-237°C. It has specific optical rotation [α]_D²⁵ + 102°. It is soluble in formanide and in ethylene glycol.

2. Gentamicin Hydrochloride: It has mp 194-209°C; and specific optical rotation [α]_D²⁵ +  113°. It is found to be freely soluble in water, methanol; slightly soluble in ether; and practically insoluble in other organic solvents.

Uses

1. It is currently the most important drug of choice for the treatment of infections caused by most aerobic Gram-negative bacteria, besides several strains of Staphylococci.

2. It essentially exhibits a broad-spectrum antibacterial activity.

3. It is found to be specifically effective against Pseudomonas, because species of this genus resistant to ‘other antibiotics’ have proved to be an important cause of surgical infections. In the same vein, gentamicin, is also very effective in severe burned-skin patients i.e., third-degree burns; and severe UTI* infections, both caused by Pseudomonas.

4. It is employed topically in the treatment of impetigo, infected bed sores, burns and nasal staphylococcal carrier state, pyodermata and also in the infections of external-eye.

Note: Because of gentamicin’s systemic toxicity, its present systemic usage is restricted and limited to life-threatening infections produced by Citrobacter, Klebsiella-Enterobacter-Serratia, Proteus and Pseudomonas. To cause an effective control it is invariably combined along with either penicillin or cephalosporin.

3.1.3 Kanamycin

Biological Sources Kanamycin is an ‘antibiotic complex’ produced by Streptomyces kanamyceticus Okami & Umezawa from the Japanese soil.** The antibiotic complex is comprised of three distinct components, namely: kanamycin A—representing the major component, and usually designated as kanamycin; besides two minor components (congeners more precisely) usually known as kanamycins B and C.

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* UTI = Urinary tract infections.

** Umezawa et al., J. Antibiot. 10A, 181 (1957); US patent 2, 931, 798 (1960).

Chemical Structure

Interestingly, these three antibiotics essentially comprise of two aminosugars (i.e., 6-amino-6- deoxy-D-glucose) which are linked individually to one single 2-deoxystreptamine aglycone (i.e., non-sugar) residue.

Characteristic Features The characteristic features of all the three kanamycins and their respective salts shall be discussed as under:

(a) Kanamycin A: [C₁₈H₃₆N₄O₁₁]; O-3-Amino-3-deoxy-a-D-glucopyranosyl-(1→6)-O-[6-amino-6-deoxy-α-D-glucopyranosyl-(1→4)-2-deoxy-D-streptamine.

It is obtained as crystals from a mixture of methanol and ethanol. It has specific optical rotation [α]_D²⁴ + 146° (0.1N. H₂SO₄).

Kanamycin A Sulphate [Synonyms: Cantrex; Crystalomicina; Enterokanacin; Kamycin; Kamynex; Kanabristol; Kanacedin; Kanamytrex; Kanasig; Kanatrol; Kanicin; Kannasyn; Kantres; Kantrox; Klebcil; Otokalixin; Resistomycin; Ophthalmokalixan; Kantrexil; Kano; Kanesein; Kanaqua.]

It is obtained as irregular prisms that decompose over a wide range above 250°C. It is freely soluble in water; and almost insoluble in nonpolar solvents and the common alcohols.

Note: USP-requires that Kanamycin A sulphate contains not less than 75% Kanamycin A on an anhydrous basis.

(b) Kanamycin B: [C₁₈H₃₇N₅O₁₀]: [Synonyms: Bekanamycin; Aminodeoxy-kanamycin; NK-1006].

It is obtained as crystals having mp 178-182°C (dec.).

It has specific optical rotations [α]_D¹⁸ + 130° (C = 0.5 in H₂O); [α]_D²¹ + 114° (C = 0.98 in H₂O). It is found to be soluble in water, formamide; slightly soluble in chloroform, isopropanol; and practically insoluble in the common alcohols and nonpolar solvents.

Kanamycin B Sulphate [Synonyms: Coltericin; Kanendomycin; Kanendos.]

(c) Kanamycin C: [C₁₈H₃₆N₄O₁₁]: It is obtained as crystals from methanol + ethanol which get decomposed above 270°C. It has specific optical rotation [α]²⁰_D + 126° (H₂O). It is found to be soluble in water; slightly soluble in formamide; and practically insoluble in nonpolar solvents and the common alcohols.

3.1.4 Neomycin

Synonyms Fradiomycin; Mycifradin; Neomin; Neolate; Neomas; Pimavecort; Vonamycin Powder V.

Biological Source It is an ‘antibiotic complex’ comprised of neomycins A, B and C. It is obtained from Streptomyces fradiae*.

Chemical Structure

Neomycin is usually obtained as a mixture of neomycin B (Framycetin) and its epimer neomycin C, the latter constitutes 5-15% of the mixture. Interestingly, in contrast to the other clinically useful aminoglycosides, neomycin is observed to comprise essentially of three sugar residues strategically attached to 2-deoxystreptamine as shown above. One of the three sugars present is the D-ribose (a common sugar).

Characteristic Features

(a) Neomycin Complex: It is an amorphous base. It is soluble in water, methanol and acidified ethanol; and almost insoluble in common organic solvents.

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* Waksman and Lechevalier, Science, 109, 305 (1949).

(b) Neomycin A: [Synonym: Neamine]: C₁₂H₂₆N₄O₆: It is obtained as crystals either from water or aqueous ethanol that get decomposed at 225-226°C. It has specific optical rotation [α]_D²⁵ + 112.8° (C = 1)

Neomycin A Hydrochloride: (C₁₂H₂₆N₄O₆.4HCl): It is obtained as an amorphous powder decomposing between 250-260°C. Its specific optical rotation is [α]_D²⁵ + 83° (C = 1).

Neomycin A, N-Acetyl Derivative: [C₁₂H₂₆N₄O₆.(CH₃CO)₄]: It is obtained as crystals from methanol having mp 334-336°C. It has specific optical rotation [α]_D²⁵ + 87°C (C = 1).

(c) Neomycin B: [Synonyms: Antibiotique EF 185; Framycetin; Enterfram; Framygen; Soframycin; Actilin;] (C₂₃H₄₆N₆O₁₃): It yields on hydrolysis neomycin A and neobiosamine B.

Neomycin B Hydrochloride: It is an amorphous white powder having specific optical rotation [α]_D²⁰ + 57° (H₂O). Its solubility in mg ml–1 at ~ 28°C: water 15.0; methanol 5.7; ethanol 0.65; isopropanol 0.05; isoamyl alcohol 0.33; cyclohexane 0.06; benzene 0.03; and is almost insoluble in acetone, ether, other organic solvents.

Neomycin B Sulphate: [Synonyms: Biosol, Bykomycin; Endomixin; Fraquinol; Myacine;

Neosulf; Neomix; Neobreltin; Nivemycin, Tuttomycin;]. It is an amorphous white powder which is almost tasteless. It has specific optical rotation [α]_D²⁰ + 54° (C = 2 in H₂O). Its solubility in mg ml–1 ~ 28°C: water 6.3; methanol 0.225; ethanol 0.095; isopropanol 0.05; and almost insoluble in acetone, ether, chloroform. The aqueous solutions are quite stable between a pH 2 to 9. The highly purified preparations are very stable to alkali, but unstable to acids. On being refluxed with Ba(OH)₂ for 18 hours it exhibited no loss of activity. On boiling with mineral acids it gives rise to furfural (an aldehyde), and also an organic base.

(d) Neomycin C: [C₂₃H₄₆N₆O₁₃]: It yields on hydrolysis neomycin A (i.e., neamine) and neobiosamine C.

Uses

1. It has good activity against Gram-positive and Gram-negative bacteria, but is very ototoxic. Therefore, its usages has been severely restricted to the oral treatment of intestinal infections.

Note: It is poorly absorbed from the digestive tract.

2. It also finds its enormous use in topical applications, such as: eardrops, eyedrops, and ointments.

3.1.5 Netilmicin

Synonyms 1-N-Ethylsisomicin; Sch-20569.

Biological Sources Sisomicin is known to be the dehydro analogue of Gentamicin C1a (see section 3.1.2), and is produced by cultures of Micromonospora inyoensis. Nevertheless, the semisynthetic N-ethyl derivative, netilmicin, is mostly used medicinally because it has an almost identical activity to gentamicin, but produces significantly much less ototoxicity.

Chemical Structure

Characteristic Features It has specific optical rotation [α]_D²⁶ + 164° (C = 3 in H₂O).

Netilmicin Sulphate [(C₂₁H₄₁N₅O₇)₂.5H₂SO₄] [Synonyms: Certomycin; Nettilin; Netilyn;

Netromicine; Netromycin; Nettacin; Vectacin; Zetamicin.]

Preparation It is a semi-synthetic derivative of sisomycin and is skillfully prepared by ethylation of the amino group in the 1-position of the 2-deoxy-streptamine ring.

Characteristic Features It is an off-white powder; pH (1 in 25 solution) ranges between 3.5-5.5; and pKa 8.1. It is found to be very soluble in water.

Uses Its antibiotic profile is very similar to that of Gentamicin.

3.1.6 Streptomycin

Synonym Streptomycin A.

Biological Sources After the qualified success and the overwhelming recognition of the therapeutic potential of penicillin an extensive and intensive search for other antibiotic substances gathered a tremendous momentum and stimulation. A major target and goal was the discovery of such antibiotics that are antagonistic to the Gram-negative microorganisms. Streptomycin was obtained from a strain of Streptomyces griseus (Krainsky) Waksman et Henrici (Actinomycetaceae); and produced by the soil Actinomycete.

Chemical Structure

Streptomycin has essentially two sugar components, namely: L-streptose and 2-deoxy-2-methylamino-L-glucose, which are linked to a non-sugar moiety streptidine evidently through two ether-linkages.

Characteristic Features Streptomycin is normally available as the trichloride, trichloride-calcium chloride double salt, phosphate or sesquisulphate, which invariably occur as powder or granules. It is more or less odourless but possesses a slightly bitter taste. It has been observed that most of its salts are hygroscopic and deliquesce on exposure to air; however, they are not affected by air or light.

Nevertheless, the salts are very soluble in water; and practically insoluble in ether, ethanol and chloroform. The solutions of its salts are levorotatory.

(a) Streptomycin Trichloride: [C₂₁H₃₉N₇O₁₂.3HCl] [Synonym: Streptomycin Hydrochloride]:

It has specific optical rotation [α]_D²⁵ – 84°. Its solubilities in mg ml–1 at ~ 28°C are: water > 20; methanol > 20; ethanol 0.90; isopropanol 0.12; isoamyl alcohol 0.117; petroleum ether 0.02; ether 0.01; and carbon tetrachloride 0.042.

(b) Streptomycin Trihydrochloride—Calcium Chloride Double Salt: [(C₂₁ H₃₉N₇O₁₂.3HCl)₂.CaCl₂] [Synonym: Streptomycin hydrochloride-Calcium chloride complex]: It is prepared from the streptomycin trihydrochloride salt. It is highly hygroscopic in nature, and gets decomposed at ~ 200°C. Its specific optical rotation [α]_D²⁵ – 76°.

(c) Streptomycin Sesquisulphate: [(C₂₁H₃₉N₇O₁₂)₂.3H₂SO₄]: [Synonyms: Streptomycin sulphate; Agristrep; Streptobrettin; Vetstrep]: It is a white to light gray or pale buff powder having faint amine-like odour. It solubilities in mg ml–1 at ~ 28°C are: water > 20; methanol 0.85; ethanol 0.30; isopropanol 0.01; petroleum ether 0.015; ether 0.035; and carbon tetrachloride 0.035.

Uses

1. It is a potent antibacterial, and more so as a tuberculosstatic agent. The MIC of streptomycin for M. tuberculosis is nearly 0.5 mcg ml–1; whereas many sensitive Gram-negative bacteria have MICs in the range of 2-4 mcg ml–1.

2. Streptomycin exerts its action in the control and management of Yersinia pestis (plague) and Francisella tularensis (tularemia); and in such typical incidences, it is invariably combined with a sulphonamide drug. Mostly hyphenated therapeutic approaches are practised, such as: streptomycin-penicillin used for endocarditis*; and streptomycin-tetracyclin employed for brucellosis.**

Note: The incidence of serious auditory impairment is now recognized and established to be far greater with dihydro-streptomycin than the parent drug streptomycin.

3. Streptomycin exerts bacteriostatic action in low concentrations and bactericidal in high concentrations to a good number of Gram-negative and Gram-positive microorganisms.

4. It is an alternate choice drug in the treatment of chancroid***, rat-bite fevers (Spirillum and Streptobacillus).

Biosynthesis of Components of Streptomycin The so called ‘components of streptomycin’ essentially comprise of two major portions, namely: first—the streptidine moiety; and secondly – the streptobiosamine, which is, in fact a disaccharide, that consists of the two sugar residues viz., streptose plus 2-deoxy-2-methylamino-L-glucose. It has been revealed through an elaborated biosynthetic studies that all the three aforesaid ‘components of streptomycin’ are derived exclusively from D-glucose. As on date no exact scientific evidence is available which may give an ample proof about the point of attachment of the three different components in the streptomycin molecule. Further, there is scanty and paucity of an elaborated explanation or information with regard to the manner whereby the individual moieties present in an aminoglycoside antibiotic. However, based on the ground realities derived from the metabolic relationships of glucose to the different moieties could be gathered and prevailed, upon directly from the various biosynthetic origins of the ‘components of streptomycin’ as shown under.

Sailent Features The salient features of the biosynthesis of components of streptomycin are, namely:

1. Streptose: D-Glucose is first converted to 4-hexosulose which on being subjected to transannular rearrangement gives rise to streptose.

2. Streptidine: D-Glucose upon demethylation yields myoinositol which upon amination produces streptamine. The resulting streptamine in the presence of L-arginine ultimately affords the formation of streptidine.

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* Endocarditis: Inflammation of the lining membrane of the heart.

** Brucellosis: A widespread infection febrile disease affecting mostly cattle, goats, swine, and sometimes humans. In

humans, it is called brucellosis or malta fever and is caused by several Brucella species.

*** Chancroid: A highly infectious nonsyphlitic ulcer; and is caused by Haemophilus ducreyi—a Gram-negative bacillus.

3. 2-Deoxy-2-methylamino-L-glucose: D-Glucose through deoxidation and methylamination yields 2-deoxy-2-methylamino-L-glucose.

3.1.7 Tobramycin

Synonyms Nebramycin Factor 6; NF 6; Gernebcin; Tobracin; Tobradistin; Tobralex; Tobramaxin; Tobrex;

Biological Sources It is a single factor antibiotic comprising about 10% of nebramycin, previously known as tenebrimycin, tenemycin; and the aminoglycosidic antibiotic complex produced by

Streptomyces tenebrarius.

Chemical Structure

Tobramycin essentially contains two aminosugar residues namely: nebrosamine and 3-deoxy-3-amino-D-glucose, and a 2-deoxystreptamine moiety. It has been found to be structurally related to kanamycin B (section 3.1.3); but evidently differs only in the absence of the 3-hydroxyl group present in the kanasamine residue.

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Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar