Action of Macrolide Antibiotics

In this chapter, the molecular-biological mode of action of macrolide antibiotics and the biochemical and genetic mechanisms of resistance to MLS antibiotics are reviewed. Based on a recent X-ray crystallographic study on a 50S ribosomal subunit from Haloarcula marismortui and the finding of intracellular macrolide accumulation, the mode of action from the viewpoint of a new hypothetical concept, deposition binding, and mechanisms of drug resistance in clinically isolated bacteria are discussed. In addition, recent major developments in macrolide antibiotics are briefly described. 

At a high concentration (1 mg/ml, equivalent to 1.4 x 10 M), erythromycin inhibits the growth of E. coli strain K cells and hardly interferes with the synthesis of nucleic acids but greatly interferes with the production of proteins in cells [5]. 

It has been found that macrolide-susceptible bacteria such as Bacillus subtilis and S. aureus accumulate about 10 times more erythromycin than do resistant bacteria, namely, clinically isolated S. aureus [6], spontaneously mutated B. subtilis [7], and intrinsically resistant . coli [8]. 

Observation of the inhibition of protein synthesis [5] and increased macrolide accumulation [6, 9] due to the binding of the antibiotics to ribosome prompted researchers to investigate more detailed mechanisms of the actions of drugs. Many well-organized reviews on the action mechanisms of the drugs have been published from thel960s until the 1990s [2, 3, 10-19]. 

The decreased accumulation of macrolide antibiotics in resistant gram-positive bacteria is discussed in detail in Section III. 

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The mode of action of macrolide antibiotics involves the inhibition of protein synthesis of specific binding to the 50S ribosomal subunit but without a specific target at the 23S ribosomal subunit and various proteins [66]. Nevertheless, the exact interaction of the macrolide and the ribosome unit is still not fully understood. In principle, the macrolide antibiotic should inhibit also mammalian mitochondrial protein syuithesis but they are unable to penetrate the mitochondrial membrane. 

Menninger and Otto [101] proposed a major inhibitory mechanism common to probably all macrolide antibiotics. In E. coli mutants with temperature-sensitive peptidyl-tRNA hydrolase (aminoacyl-tRNA hydrolase EC 3.1.1.29), they observed that peptidyl-tRNA accumulates at a nonpermissive temperature (40°C) and that the cells die. The accumulation at a high temperature was enhanced when the cells were pretreated with erythromycin, carbomycin, or spiramycin at doses sufficient to inhibit protein synthesis in wild-type cells but not sufficient to kill either mutant or wild-type cells at the permissive temperature (30°C). Based on their observations, they suggested that stimulated dissociation of peptidyl-tRNA from ribosomes is the major mechanism of action of macrolide antibiotics. Their observations agree with recent results showing that a macrolide antibiotic binds to peptidyltransferase in ribosome. 

According to Nissen et al. [27], proteins L4, L22, and L39e (the letter e represents a protein in a bacterial SOS ribosomal subunit, a protein that belongs to one of the homologs in eukaryotic 60S ribosomal subunit) have been shown to be present in the polypeptide exit tunnel [average diameter, about IS A length of the tunnel, 100 A (Fig. 3B)] present in the SOS ribosomal subunit from H. maris-mortui. Six other proteins (L19, L22, L23, L24, L29, and L31e) are known to be located in the exit area of the polypeptide tunnel [26, 27]. If these proteins are involved in the inhibitory actions of macrolide antibiotics, mutant bacteria resistant to macrolides will develop in the future. 

The molecular biological mode of action of macrolide antibiotics and the biochemical and genetic mechanism of resistance to macrolide, lincosamide, and type B streptogramin antibiotics were reviewed in this chapter. 

An interesting exception to the absolute validity of the tifth postulate is the considerable activity of chloramphenicol derivatives in cell-free model systems of protein synthesis when these derivatives are substituted with amino acyl residues instead of with dichloroacetyl as is the antibiotic itself (rev. in 2°)). This has been traced to the necessity of the dichloroacetyl grouping in aiding in the permeation of the antibiotic through the bacterial envelope 21 The amino acyl derivatives have very low antibacterial activity 20. Permeation failures of actinomycin D, macrolides and distamycin A with respect to certain families of bacteria occlude the action of these antibiotics on their intracellular drug receptors and target reactions but can be overcome experimentally by measures which render test organisms permeable. 

This class of macrolide antibiotic has mostly antiparasitic activity. Avermectin Bia (45) and ivermectin (46) (O Scheme 18) are used mostly in veterinary medicine, however, some semisynthetic derivatives are also used for treatment of onchocerciasis in humans [59]. The action of avermectin is believed to stimulate specific chloride ion transport systems increasing the membrane permeability to Cl ions via GABA (y-butyrate) receptors and non-GABA receptors [60]. 

Er hromycin Sulfisoxazole (Eryzole, Pediazole) [Anti-infective, Macrolide/Sulfonamide] Uses Upper lower resp tract bacterial Infxns H. influenzae otitis media in children Infxns in PCN-allergic pts Action Macrolide antibiotic w/ sulfonamide Dose Adults. Based on erythromycin content 400 mg erythromycin/1200 mg sulfisoxazole PO q6h Feds > 2 mo. 40-50 mg/kg/d erythromycin 150 mg/kg/d sulfisoxazole PO -s- q6h max 2 g/d erythromycin or 6 g/d sulfisoxazole x 10 d in renal impair Caution [C (D if near term), +] w/ PO anticoagulants, hypoglycemics, phenytoin, cyclosporine Contra Infants <2 mo Disp Susp SE GI upset Additional Interactions T Effects of sulfonamides W/ ASA, diuretics, NSAIDs, probenecid EMS See Erythromycin OD See Erythromycin... 

Rapamycin (sirolimus), a macrolide antibiotic, has been used recently in organ transplantation for its potent immunosuppressive actions by inhibiting both cytokine mediated and growth factor mediated proliferation of smooth muscle cells and lymphocytes [55, 56]. In the RAVEL trial of non-acute single vessel lesions, the Sirolimus-eluting stent was compared to bare metal stent (BMS) in a 1 1 fashion [57]. One-year major adverse cardiovascular events and 6 month neointimal proliferation as assessed by late luminal loss (-0.01 0.33 mm in Sirolimus stent versus 0.80 0.53 mm in BMS) were improved. The Sirolimus-eluting stent thus virtually eliminated in-stent restenosis with no evidence of edge effect, dissection, or in-stent thrombosis. 

Although it is not chemically related to cyclosporine, tacrolimus (6.7) has a similar mechanism of action. Tacrolimus is an immunosuppressant macrolide antibiotic derived from Streptomyces tsukubaenis. Like cyclosporine, tacrolimus inhibits the same cytoplasmic phosphatase, calcineurin, which catalyzes the activation of a T-cell-specific transcription factor (NF-AT) involved in the biosyntheses of interleukins such as IL-2. Sirolimus (6.8) is a natural product produced by Streptomyces hydroscopicus, it blocks the ability of T cells to respond to cytokines. 

Tacrolimus (FK 506) is an immunosuppressant macrolide antibiotic produced by Streptomyces tsukubaensis. It is not chemically related to cyclosporine, but their mechanisms of action are similar. Both drugs bind to cytoplasmic peptidyl-prolyl isomerases that are abundant in all tissues. While cyclosporine binds to cyclophilin, tacrolimus binds to the immunophilin FK-binding protein (FKBP). Both complexes inhibit calcineurin, which is necessary for the activation of the T-cell-specific transcription factor NF-AT. 

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