Antimicrob Agents Chemother. 1990 Jun;34(6):1056-60.
Intracellular accumulation of azithromycin [Zithromax] by cultured human fibroblasts.

Gladue RP, Snider ME.

Central Research Division, Pfizer, Inc., Groton, Connecticut 06340.

Azithromycin [Zithromax] was shown to achieve high concentrations in human skin fibroblasts. Intracellular penetration occurred rapidly (10 micrograms/mg of cellular protein after 3 h) and then increased progressively over a 3-day period; azithromycin [Zithromax] accumulated up to 21 times more than erythromycin (61.1 versus 2.9 micrograms/mg of protein). Uptake was dependent on the extracellular concentration, was inhibited at 4 degrees C, did not occur in nonviable cells, and was reduced by a low pH. Intracellular accumulation was not affected by the metabolic inhibitor 2,4-dinitrophenol or sodium fluoride or by the nucleoside transport inhibitor 2-chloradenosine. Once concentrated in cells, azithromycin [Zithromax] remained intracellular and was released slowly in the absence of extracellular drug, compared with erythromycin (17 versus 78% released after 1 h). After 48 h of incubation in drug-free medium, 27% of the initial amount of azithromycin [Zithromax] remained cell associated. The release of azithromycin [Zithromax] was not affected by various monokines reported to stimulate fibroblasts (interleukin-1 or tumor necrosis factor) or by exposure to bacteria. Incubation of azithromycin-loaded fibroblasts with human polymorphonuclear leukocytes resulted in a higher intracellular accumulation of azithromycin [Zithromax] in polymorphonuclear leukocytes than in cells incubated with free nonintracellular azithromycin [Zithromax] for the same time (8.3 versus 2.2 micrograms/ml after 2 h), suggesting a more efficient or rapid uptake through cell-to-cell interaction. The widespread distribution of fibroblasts in tissues suggests a potential for these cells, and possibly other lysosome-containing tissue cells, to serve as a reservoir for azithromycin, slowly releasing it for activity against extracellular organisms at sites of infection and passing it to phagocytes for activity against intracellular pathogens and potential transport to sites of infection.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2168141&dopt=Abstract Zithromax azithromycin




Arzneimittelforschung. 1990 Jun;40(6):686-9.
Mechanism of azithromycin [Zithromax] uptake in human polymorphonuclear leucocytes.

Laufen H, Wildfeuer A, Lach P.

Research and Development Division, Heinrich Mack Nachf., Illertissen, Fed. Rep. of Germany.

The new antibiotic azithromycin [Zithromax] (CP-62.993) is enriched in human polymorphonuclear leucocytes by up to 300-fold the extra-cellular concentrations. To approach an understanding of the underlying mechanism of this unique behavior of azithromycin, some characteristics of the uptake process were investigated in vitro. The speed of the uptake in human polymorphonuclear leucocytes was found to be independent of the extracellular starting concentrations of azithromycin. There was no indication of saturation up to extracellular concentrations of 100 micrograms/ml. The uptake was largely determined by incubation temperatures. At 4 degrees C no penetration into the cells could be observed. The activation energy of azithromycin [Zithromax] uptake came to 144 kJ mol-1, twice the value of erythromycin uptake. The presence of various inhibitors of cell metabolism did not change the intracellular accumulation compared to control, nor did the presence of some agents which interfere with certain transport channels. These findings suggest that passive diffusion is an essential mechanism of azithromycin [Zithromax] transport through the phagocyte membrane, while active transport is less important. The high cellular enrichment of azithromycin [Zithromax] and the relatively high activation energy of the uptake process could be explained by accumulation of the drug in phagocytic lysosomes and this would also be in keeping with pH partition considerations.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2168704&dopt=Abstract Zithromax azithromycin




Antimicrob Agents Chemother. 1987 Dec;31(12):1939-47.
Spectrum and mode of action of azithromycin [Zithromax] (CP-62,993), a new 15-membered-ring macrolide with improved potency against gram-negative organisms.

Retsema J, Girard A, Schelkly W, Manousos M, Anderson M, Bright G, Borovoy R, Brennan L, Mason R.

Central Research Division, Pfizer Inc., Groton, Connecticut 06340.

The macrolide antibiotic azithromycin [Zithromax] (CP-62,993; 9-deoxo-9a-methyl-9a-aza-9a-homoerythromycin A; also designated XZ-450 [Pliva Pharmaceuticals, Zagreb, Yugoslavia]) showed a significant improvement in potency against gram-negative organisms compared with erythromycin while retaining the classic erythromycin spectrum. It was up to four times more potent than erythromycin against Haemophilus influenzae and Neisseria gonorrhoeae and twofold more potent against Branhamella catarrhalis, Campylobacter species, and Legionella species. It had activity similar to that of erythromycin against Chlamydia spp. Azithromycin [Zithromax] was significantly more potent versus many genera of the family Enterobacteriaceae; its MIC for 90% of strains of Escherichia, Salmonella, Shigella, and Yersinia was less than or equal to 4 micrograms/ml, compared with 16 to 128 micrograms/ml for erythromycin. Azithromycin [Zithromax] inhibited the majority of gram-positive organisms at less than or equal to 1 micrograms/ml. It displayed cross-resistance to erythromycin-resistant Staphylococcus and Streptococcus isolates. It had moderate activity against Bacteroides fragilis and was comparable to erythromycin against other anaerobic species. Azithromycin [Zithromax] also demonstrated improved bactericidal activity in comparison with erythromycin. The mechanism of action of azithromycin [Zithromax] was similar to that of erythromycin since azithromycin [Zithromax] competed effectively for [14C]erythromycin ribosomebinding sites.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2449865&dopt=Abstract Zithromax azithromycin




Antimicrob Agents Chemother. 1989 Mar;33(3):277-82.
In vitro and in vivo uptake of azithromycin [Zithromax] (CP-62,993) by phagocytic cells: possible mechanism of delivery and release at sites of infection.

Gladue RP, Bright GM, Isaacson RE, Newborg MF.

Central Research Division, Pfizer, Inc., Groton, Connecticut 06340.

Azithromycin, a novel azalide antibiotic, concentrated in human and mouse polymorphonuclear leukocytes (PMNs), murine peritoneal macrophages, and mouse and rat alveolar macrophages, attaining intracellular concentrations up to 226 times the external concentration in vitro. In murine peritoneal macrophages, azithromycin [Zithromax] achieved concentration gradients (internal to external) up to 26 times higher than erythromycin. The cellular uptake of azithromycin [Zithromax] was dependent on temperature, viability, and pH and was decreased by 2,4-dinitrophenol. Azithromycin [Zithromax] did not decrease phagocyte-mediated bactericidal activity or affect PMN or macrophage oxidative burst activity (H2O2 release or Nitro Blue Tetrazolium reduction, respectively). Azithromycin [Zithromax] remained in cells for several hours, even after extracellular drug was removed. However, its release was significantly enhanced by phagocytosis of Staphylococcus aureus (82 versus 23% by 1.5 h). In vivo, 0.05 micrograms of azithromycin [Zithromax] was found in peritoneal fluids of mice 20 h after oral treatment with a dose of 50 mg/kg. Following caseinate-induced PMN infiltration, there was a sixfold increase in peritoneal cavity azithromycin [Zithromax] to 0.32 micrograms, most of which was intracellular. Therefore, the uptake, transport, and later release of azithromycin [Zithromax] by these cells demonstrate that phagocytes may deliver active drug to sites of infection.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2543276&dopt=Abstract Zithromax azithromycin




Eur J Clin Microbiol Infect Dis. 1989 Jun;8(6):544-9.
Quality control parameters and interpretive criteria for in vitro susceptibility tests with the macrolide azithromycin. Collaborative Antimicrobial Susceptibility Testing Group.

Barry AL, Thornsberry C, Gavan TL.

Clinical Microbiology Institute, Tualatin, Oregon 97062.

Quality control parameters for broth microdilution and disk diffusion susceptibility tests were defined and the interpretive criteria for disk diffusion tests reviewed. For interpretation of tests with 15 micrograms azithromycin [Zithromax] disks, the following criteria are recommended: greater than or equal to 19 mm for the susceptible category (MIC less than or equal to 2.0 micrograms/ml) and less than or equal to 15 mm for the resistant category (MIC greater than or equal to 8.0 micrograms/ml). Using these criteria, there was 97% overall agreement between broth dilution and disk diffusion tests; Haemophilus influenzae isolates were susceptible to azithromycin [Zithromax] by both methods. The quality control strain Staphylococcus aureus ATCC 25923 gave zones of 21 to 26 mm in diameter in a six-laboratory collaborative study. In azithromycin [Zithromax] broth microdilution tests the following MIC control limits are recommended: Escherichia coli ATCC 25922, 2.0-8.0 micrograms/ml; Staphylococcus aureus ATCC 29213, 0.25-1.0 micrograms/ml; and Enterococcus faecalis ATCC 29212, 1.0-4.0 micrograms/ml.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2548865&dopt=Abstract Zithromax azithromycin




Clin Orthop. 1999 Feb;(359):229-36.
Oral rifampin plus azithromycin [Zithromax] or clarithromycin to treat osteomyelitis in rabbits.

Shirtliff ME, Mader JT, Calhoun J.

Department of Internal Medicine, University of Texas Medical Branch, Galveston 77555-1019, USA.

A rabbit model for Staphylococcus aureus osteomyelitis was used to compare 28-day combination antibiotic therapy using oral rifampin (40 mg/kg, twice daily) plus oral azithromycin [Zithromax] (50 mg/kg, once per day), oral clarithromycin (80 mg/kg, twice daily), or parenteral nafcillin (30 mg/kg, four times daily). The left tibial metaphysis of New Zealand White rabbits was infected with Staphylococcus aureus. Grades 3 to 4 osteomyelitis (according to the Cierny-Mader classification system) development in the rabbits was confirmed radiographically. After antibiotic therapy regimens of 28 days, all tibias from controls that were infected but left untreated (n = 10) revealed positive cultures for Staphylococcus aureus at a mean concentration of 2.8 x 10(4) colony forming units/g bone. The rifampin plus clarithromycin (n = 15) and rifampin plus azithromycin [Zithromax] (n = 15) groups showed significantly lower percentages of positive Staphylococcus aureus infection (20% and 13.3%, respectively) and bacterial concentrations (3.5 x 10(1) and 1.75 x 10(1) colony forming units/g bone, respectively). The osteomyelitic tibias of the nafcillin plus rifampin treated group (n = 7) showed no detectable Staphylococcus aureus infection (significantly lower than controls). The differences observed for bone bacterial concentrations and sterilization percentages between the antibiotic treated groups were not statistically significant. Although fluoroquinolones (including ofloxacin and ciprofloxacin) are the agents usually prescribed with rifampin, increasing resistance has been observed. Although macrolides traditionally are not used in the treatment of osteomyelitis, the results of this study indicate that azithromycin [Zithromax] and clarithromycin may be attractive partners for rifampin for the treatment of Staphylococcus aureus osteomyelitis in humans.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10078148&dopt=Abstract Zithromax azithromycin