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The macrolide antibiotics typically have a large lactone ring in their structure and are much more effective against gram-positive than gram-negative bacteria. They are also active against mycoplasmas and some rickettsiae. (See also Polyene Macrolide Antibiotics.)


Macrolides fall into three classes, depending on the size of the macrocyclic lactone ring. None of the 12-membered ring group is used clinically. Erythromycin and the closely related oleandomycin and troleandomycin belong to the 14-membered ring group. Azithromycin (synthesized from erythromycin) and gamithromycin are 15-ring members, a subclass referred to as azalides. Of the 16-membered ring group, spiramycin, josamycin, tylosin, and tilmicosin (synthesized from tylosin), are used clinically. Tulathromycin contains three amine rings and is classified as a triamilide. Ketolides, which include tylosin and spiramycin, are closely related macrolides.

General Properties

A macrolide is actually a complex mixture of closely related antibiotics that differ from one another with respect to the chemical substitutions on the various carbon atoms in the structure and in the aminosugars and neutral sugars. For example, erythromycin is mostly erythromycin A, but B, C, D, and E forms may also be included in the preparation. The macrolide antibiotics are colorless, crystalline substances. They contain a dimethylamino group, which makes them basic. Although they are poorly water soluble, they do dissolve in more polar organic solvents. Macrolides are often inactivated in basic (pH >10) as well as acidic environments (pH <4 for erythromycin). The multiple functional groups make it possible for them to undergo a large number of chemical reactions. More stable ester forms, eg, acetylates, estolates, lactobionate, succinates, propionates, and stearates, are commonly used in pharmaceutical preparations.

Antimicrobial Activity

The antimicrobial mechanism seems to be the same for all of the macrolides. They interfere with protein synthesis by reversibly binding to the 50S subunit of the ribosome. They appear to bind at the donor site, thus preventing the translocation necessary to keep the peptide chain growing. The effect is essentially confined to rapidly dividing bacteria and mycoplasmas. Macrolides are regarded as being bacteriostatic but demonstrate bactericidal activity at high concentrations. Macrolides are significantly more active at higher pH ranges (7.8–8). Macrolides are considered to be time dependent in terms of antimicrobial efficacy.

The macrolides appear to have immunomodulatory effects useful to treat respiratory infections, in particular, those associated with Pseudomonas aeruginosa, based on efficacy at doses (concentrations) considered ineffective against susceptible bacteria.

Lack of cell wall permeability renders most gram-negative organisms inherently resistant to macrolides. There are a few exceptions, and gram-negative forms without cell walls are usually susceptible. Resistance to macrolides in gram-positive organisms results from alterations in ribosomal structure (target site methylation or mutation) and loss of macrolide affinity. Post-translational methylation results in cross-resistance to lincosamides and streptogramins. Macrolide resistance may be intrinsic or plasmid-mediated and constitutive or inducible; it may develop rapidly (erythromycin) or slowly (tylosin) and generally results in cross-resistance between macrolides. Efflux from cells is a second important mechanism of resistance for some members of this class, as is, less frequently, drug inactivation.

Macrolides are active against most aerobic and anaerobic gram-positive bacteria, although there is considerable variation as to potency and activity. In general, macrolides are not active against gram-negative bacteria, but some strains of Pasteurella, Haemophilus, and Neisseria spp may be sensitive. Exceptions include tilmicosin, gamithromycin, and tulathromycin, for which the spectra are characterized as broader and include Mannheimia haemolytica and Pasteurella multocida, as well as the above-mentioned gram-negative bacteria. Helicobacter also is generally included in the spectrum. Azithromycin, derived from erythromycin, includes Bordetella in its spectrum. Bacteroides fragilis strains are moderately susceptible to macrolides. Macrolides are active against atypical mycobacteria, Mycobacterium, Mycoplasma, Chlamydia, and Rickettsia spp but not against protozoa or fungi. In vitro synergism is seen with cefamandole (against B fragilis), ampicillin (against Nocardia asteroides), and rifampin (against Rhodococcus equi).

Pharmacokinetic Features

Macrolides are readily absorbed from the GI tract if not inactivated by gastric acid. Oral preparations are often enteric-coated, or stable salts or esters (such as stearate, lactobionate, glucoheptate, propionate, and ethylsuccinate) are used. Plasma concentrations peak within 1–2 hr in most cases, although absorption patterns may be erratic because of the presence of food and may depend on the salt or ester used. Absorption from the ruminoreticulum is usually delayed and is unreliable. Erythromycin and tylosin may also be administered IV or IM. Tilmicosin, gamithromycin, and tulathromycin are administered SC, except in swine, for which an oral tilmicosin preparation is available. Absorption after injection is rapid, but pain and swelling can develop at the injection sites.

Macrolides become widely distributed in tissues, and concentrations are about the same as in plasma, or even higher in some instances. They actually accumulate within many cells, including macrophages, in which they may be ≥20 times the plasma concentration. WBCs will then facilitate distribution to the site of inflammation. This accumulation accounts in part for the long dosing interval that characterizes some macrolides (eg, tilmicosin). With spiramycin, the tissue concentrations remain especially high, even though plasma concentrations are rather low. Macrolides tend to concentrate in the spleen, liver, kidneys, and particularly the lungs. They enter pleural and ascitic fluids and concentrate in the eye but do not distribute to the eye or the CSF (only 2%–13% of plasma concentration unless the meninges are inflamed). They concentrate in the bile and milk. Up to 75% of the dose is bound to plasma proteins, and they bind to α1-acid glycoproteins rather than to albumin.

Metabolic inactivation of the macrolides is usually extensive, but the relative proportion depends on the route of administration and the particular antibiotic. After administration PO, 80% of an erythromycin dose undergoes metabolic inactivation, whereas tylosin appears to be eliminated in an active form.

Macrolide antibiotics and their metabolites are excreted mainly in bile (>60%) and often undergo enterohepatic cycling. Urinary clearance may be slow and variable (often <10%) but may represent a more significant route of elimination after parenteral administration. For example, in people, 14% of azithromycin and 20%–40% of clarithromycin is excreted unchanged in urine. The concentration of macrolides in milk often is several times greater than in plasma, especially in mastitis.

Macrolides tend to be characterized by high oral bioavailability, but this is variable among species, drugs, and salts. For example, oral bioavailability for tylosin is 0.35 for the tartrate salt versus 0.14 for the phosphate. For azithromycin, oral bioavailability is 39% in foals 6–10 wk old, 59% in cats, and 97% in dogs. The accumulation of macrolides among different tissues contributes to the large volume of distribution (for azithromycin 12 L/kg in dogs, 23 L/kg in cats, 22 L/kg in foals 6–10 wk old) and long elimination half-life (for azithromycin, 29 hr in dogs, 35 hr in cats, and 20 hr in foals) For tulathromycin, the elimination half-life is 65 hr in calves and 69 hr in pigs 2–3 mo old. Because of these long half-lives, time to steady state may be prolonged, and a loading dose may be indicated for multiple dosing. Tylosin, however, is an exception, with a volume of distribution approximating 1 L/kg and a half-life of 1–2 hr. Another exception is azithromycin, which has a half-life in cats that varies among tissues, reaching >72 hr for some. Effective plasma inhibitory concentrations are maintained for ~8 hr after administration PO and for ~12–24 hr after IM injection. Dosage frequencies are commonly 2–3 times/day, PO, or 1–2 times/day, parenterally.

Therapeutic Indications and Dose Rates

The macrolides are used to treat both systemic and local infections. They are often regarded as alternatives to penicillins for treatment of streptococcal and staphylococcal infections. General indications include upper respiratory tract infections, bronchopneumonia, bacterial enteritis, metritis, pyodermatitis, urinary tract infections, arthritis, and others. Macrolides are indicated for treatment of Rhodococcus respiratory tract infections in foals. Formulations to treat mastitis are also available and often have the advantage of a short withholding time for milk. Tilmicosin, gamithromycin, and tulathromycin are approved for use in treatment of bovine respiratory diseases associated with Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni. In swine, tilmicosin phosphate is added to feed or water for control of swine respiratory disease.

A selection of general dosages for some macrolides is listed in Table: Dosages of MacrolidesTables. The dose rate and frequency should be adjusted as needed for the individual animal.

Table 23

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Special Clinical Concerns

Toxicity and adverse effects are uncommon for most macrolides (except tilmicosin), although pain and swelling may develop at injection sites. Hypersensitivity reactions have occasionally been seen. Erythromycin estolate may be hepatotoxic and cause cholestasis; it may also induce vomiting and diarrhea, particularly when high doses are administered. Horses are sensitive to macrolide-induced GI disturbances that can be serious and even fatal. In pigs, tylosin may cause edema of the rectal mucosa, mild anal protrusion with diarrhea, and anal erythema and pruritus. After 5 mg/kg/day, dogs had a greater tendency to develop ventricular tachycardia and fibrillation during acute myocardial ischemia. Tilmicosin is characterized by cardiac toxicity (tachycardia and decreased contractility). Parenteral (but not oral) administration should be avoided in swine, and extra-label use should be avoided. Cattle have died after IV injection of tilmicosin, and human injury is possible after accidental exposure.

Macrolide antibiotics probably should not be used with chloramphenicol or the lincosamides, because they may compete for the same 50S ribosomal binding site, although the in vivo significance of this potential interaction is unclear. Activity of macrolides is depressed in acidic environments. Macrolide preparations for parenteral administration are incompatible with many other pharmaceutical preparations. Erythromycin and troleandomycin and other macrolides are microsomal enzyme inhibitors that depress the CYP3A4 (in people) and thus the metabolism of many drugs. Macrolides also are substrates for and potentially potent inhibitors of p-glycoprotein efflux pumps.

Alkaline phosphatase, bilirubin, sulfobromophthalein (BSP®), total WBC count, eosinophil count, AST, and ALT may increase. Cholesterol concentrations may decrease.

Regulatory requirements for withdrawal times and milk discard times vary among countries. These should be followed carefully to prevent food residues and consequent public health implications. The withdrawal times listed in Table: Drug Withdrawal and Milk Discard Times of MacrolidesTables serve only as general guidelines. Tilmicosin is characterized by a 28-day withdrawal time and should not be used in any species other than adult cattle (but not in dairy cows >20 mo old).

Table 24

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Last full review/revision November 2015 by Dawn Merton Boothe, DVM, PhD

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