Anti-Microbial Drugs

Transcription

1 Name: Date: Wed. March 9 th 2011 Class: I "Pharmacology Anti-Microbial Drugs Lecture 6 د. حيدر الشكرجي

2 Fluroroquinolones (DNA gyrase inhibitors): The important quinolones are synthetic fluorinated analogs of nalidixic Acid. Introduction of the first fluorinated quinolones, norfloxacin, was rapidly followed by development of other members of this group. Mechanism of Action: The Fluoroquinolones enter the bacterium by passive diffusion through waterfilled proteins channels (porins) in the outer membrane. Once inside the cell, they inhibit the replication of bacterial DNA by interfering with the action of DNA gyrase (topoisomerase II) and topoisomerase IV during bacterial growth and reproduction. Topoisomerase II changes the configuration of DNA by nicking, pass through and resealing mechanism. Topoisomerase IV is implicated in the process of segregating newly replicated DNA. Binding of the quinolone to both the enzyme and DNA forms a ternary complex that inhibits the resealing step, and can cause cell death by inducing cleavage of the DNA.In gram-negative org. the inhibition of DNA gyrase is more significant than that of topoisomerase IV, whereas in gram-positive org., the opposite is true. Antimicrobial Spectrum: All the Fluroroquinolones are bactericidal. Like aminoglycosides, quinolones exhibit concentration dependent killing. Bactericidal activity becomes more pronounced as the serum drug concentration increases to approximately 30 fold the MIC. In general, they're effective against gram ve organisms such as Enterobacteriacea, Pseudomonas species, Haemophilus influenzae, Moraxella catarrhalis, Legionellaceae, Chlamydia and Mycobacteria (except for Mycobacterium avium intracellular complex). They're effective for the treatment of gonorrhea but not syphilis. The newer agents also have good activity against gram +ve organisms such as Streptococcus pneumonia. Some of those have activity against some anaerobes like Bacteriodes fragilis. Page 2

3 Classification of the Fluroroquinolones: 1. First generation (Nalidixic Acid): It's non-fluorinated quinolones with a narrow spectrum of susceptible microorganisms, has a moderate gram ve activity usually confined to the urinary tract. 2. Second generation (Ciprofloxacin, Norfloxacin, Ofloxacin) They have activity against systemic aerobic gram ve infections, and also have some activity against gram +ve and atypical organisms such as Chlamydia, Mycoplasma and Legionella, which spend part or all of their life cycle inside a host cell. 3. Third generation (Levofloxacin, Gatifloxacin, Sparfloxacin) (LGS) These agents retain expanded gram ve activity and show improved activity against atypical organisms and specific gram +ve bacteria. 4. Fourth generation (Moxifloxacin): This agent shows improved gram +ve coverage, maintains gram ve activity and gains anaerobic coverage page 389 Page 3

4 Clinical uses of Ciprofloxacin: Anti-Microbial Drugs 1. Urinary tract infections even when caused with multi-stage resistant bacteria such as Pseudomonas. 2. Bacterial Diarrhea caused by Shigella, Salmonella, toxigenic E. coli or Campylobacter. 3. Typhoid fever. 4. Gonorrhea due to penicillinase producing and non-penicillinase producing strains. 5. Legionellosis 6. Resistant TB. 7. Eradication of Meningococci from carriers. 8. Prophylaxis and treatment of Anthrax. 9. Resistant Respiratory infections (not pneumonia or sinusitis). 10. Pseudomonal infections associated with cystic fibrosis page 389 Resistance A. Altered target: resistance is frequently associated with mutations in both bacterial DNA gyrase and Topoisomerase IV. B. Decreased accumulation: a. Decreased number of porin proteins in the outer membrane of the resistant cell, thereby impairing access of the drug to the intracellular topoisomerase. b. Energy-dependent efflux system in the cell membrane. Page 4

5 Pharmacokinetics: 1. Absorption: Ingestion of the fluoroquinolones with sucralfate, antacids containing aluminum or magnesium, or dietary supplements containing iron, zinc, or calcium can interfere with the absorption of these antimicrobial drugs. 2. Fate: Achieved plasma levels of free norfloxacin are insufficient for treatment of systemic infections. All the fluoroquinolones distribute well into all tissues and body fluids. Penetration into CSF is low for ofloxacin. They are excreted by the renal route. Adverse Effects: 1. Gastrointestinal: the most common adverse effects are nausea, vomiting and diarrhea. 2. CNS problems: the most common CNS effects are headache and dizziness or light headedness. 3. Photosensitivity: patients taking fluoroquinolones are advised to avoid excessive sunlight and to apply sun creams. 4. Connective tissue problems: fluoroquinolones should be avoided in pregnancy, in nursing mothers, and in children under 18 years of age because articular cartilage erosion (arthropathy) occurs in immature experimental animals. However, children with cystic fibrosis who receive ciprofloxacin have had few problems. In adults, they can infrequently cause ruptured tendons. 5. Contraindications: moxifloxacin prolong the QT interval and, thus, should not be used in patients who are predisposed to arrhythmias. 6. Drug Interactions: the effect of antacids and cations on the absorption of these agents was considered above. Ciprofloxacin and ofloxacin can increase the serum levels of theopylline by inhibiting its metabolism. Folic Acid Antagonists: Page 5

6 Coenzymes containing folic acid are required for the synthesis of purines and pyrimidines (precursors of RNA and DNA) and other compounds necessary for cellular growth and replication. Therefore in the absence of folic acid, cells cannot grow or divide. Humans cannot synthesize folic acid and, thus, must obtain preformed folate as vitamin from the diet. In contrast, many bacteria are impermeable to folic acid, and therefore must rely on their ability to synthesize folate de novo. Pteridine prec.+paba folic acid tetrahydrofolic a. Sulfonamides: Sulfa drugs differ from each other not only in their chemical and physical properties but also in their pharmacokinetics. Mechanism of action: Folic acid is synthesized from p-aminobenzoic acid (PABA), pteridine, and glutamate. All sulfonamides are synthetic analogs of PABA. Because of their structural similarity to PABA, the sulfonamides compete with this substrate for the bacterial enzymes, dihydropteroate synthetase. They thus inhibit the synthesis of bacterial folic acid and, thereby, the formation of its essential cofactor forms. The sulfa drugs, including co-trimoxazole are bacteriostatic. Antibacterial Spectrum: 1. Sulfa drugs are active against selected enterobaceria in the urinary tract and nocardia. Page 6

7 2. Sulfadiazine, in combination with the dihydrofolate reductase inhibitor pyrimethamine, is the preferred form of treatment for toxoplasmosis and chloroquine-resistant malaria. 3. Many strains of formerly susceptible species, including meningococci, pneumococci, streptococci, staphylococci, and gonococci, are now resistant. Resistance: Resistance is generally irreversible and may be due to: 1. An altered dihydropteroate synthetase. 2. Decreased cellular permeability to sulfa drugs. 3. Enhanced production of natural substrate, PABA. Pharmacokinetics Sulfonamides can be divided into three major groups: 1. Oral absorbable agents: 1. Short acting agents, e.g. sulfisoxazole. 2. Medium acting agents, e.g. sulfadiazine, sulfamethoxazole. *sulfadiazine+pyrimethamine toxoplasmosis 3. Long acting agents, e.g. sulfadoxine. *sulfadoxine+pyrimethamine=(fansidar) malaria. After oral administration, most sulfa drugs are well absorbed via the small intestine. They are bound to serum albumin. Sulfa drugs penetrate well into CSF even in the absence of inflammation. They can also pass the placental barrier and enter fetal tissues. The sulfa drugs are acetylated, primarily by in the liver. The product retains the toxic potential to precipitate at neutral or acidic PH. This causes crystalluria (stone formation) and therefore potential damage to the kidney. Sulfa drugs are eliminated by glomerular filtration. The sulfonamides may also be eliminated in breast milk. Page 7

8 2. Oral non-absorbable agents: e.g. sulfasalazine. Sulfasalazine is reserved for the treatment of chronic inflammatory bowel disease (e.g. Crohn s disease, or ulcerative colitis). Local intestinal flora split sulfasalazine into sulfapyridine and 5-aminosalicylate, with the latter exerting the antiinflammatory effect. Absorption of the sulfapyridine can lead to toxicity in patients who are slow acetylators. 3. Topical agents e.g. sodium sulfacetamide, silver sulfadiazine. The former is effective treatment for bacterial conjunctivitis while the latter is preferred for prevention of infection of burn wounds. Adverse effect: 1. Crystalluria: nephrotoxicity develops as a result of crystalluria. Adequate hydration and alkalinization of urine prevent the problem by reducing the concentration of drug and promoting its ionization. Sulfisoxazole and sulfamethoxazole are less liable to cause crystalluria. 2. Hypersensitivity: hypersensitivity reactions, such as rashes, angioedema, and Stevens-Johnson syndrome, are fairly common. 3. Hemopoietic Disturbances: hemolytic anemia is encountered in patients with glucose 6-phosphate dehydrogenase deficiency. Granulocytopenia and thrombocytopenia can also occur. 4. Kernicterus: this disorder may occur in newborns because of sulfa drugs displace bilirubin from binding sites on serum albumins. The bilirubin is then free to pass into CNS, because the baby s blood brain barrier is not fully developed. 5. Drug Potentiation: transient potentiation of the effect of tolbutamide or warfarin results from their displacement from binding sites. 6. Contraindications: due to danger of kernicterus, sulfa drugs should be avoided in newborns and infants less than two months of age, as well as for pregnant women at term. Sulfonamides should not be given to patients receiving methenamine for UTI. Trimethoprim: Trimethoprim is a potent inhibitor of bacterial dihydrofolate reductase. Page 8

9 Mechanism of action: The active form of folate is tetrahydrofolate acid (THFA) that is formed through reduction of dihydrofolate reductase. This enzymatic reaction is inhibited by trimethoprim, leading to decreased availability of the THFA coenzymes required for purine, pyrimidine, and amino acid synthesis. The bacterial reductase has a much stronger affinity for trimethoprim than does the mammalian enzyme. Note: examples of other drugs that function as folate reductase inhibitors include pyrimethamine and methotrexate. Anti-bacterial Spectrum: The antibacterial spectrum of trimethoprim is similar to that of sulfamethoxazole. Resistence: Resistance in gram ve bacteria is due to the presence of altered dihydrofolate reductase that has a lower affinity to trimethoprim. Pharmacokinetics: The ½ life of trimethoprim is similar to that of sulfamethoxazole. However, because the drug is a weak base, higher concentrations of trimethoprim are achieved in the relatively acidic prostatic and vaginal fluid. The drug also penetrates the CSF. Adverse Effects: Trimethoprim can produce the effects of folic acid deficiency. These effects include megaloblastic anemia, leukopenia, and granulocytopenia. These blood disorders can be reversed by the simultaneous administration of folinic acid, which does not enter the bacteria. Page 9

10 Co-trimoxazole: The combination of trimethoprim with sulfamethoxazole (ratio 1.5) is called co-trimoxazole. The synergistic antimicrobial activity results from its inhibition of two sequential steps in the synthesis of THFA. Therapeutic Applications: 1. Pneumocystis Jiroveci Pneumonia: a common opportunistic infection caused by pneumocystis carinii complicating AIDs (given I.V.). 2. Listeriosis: septicemia and meningitis caused by listeria monocytogenes (ampicillin or co-trimoxazole). 3. Prostate and urinary tract infections: trimethoprim concentrates in prostatic and vaginal fluids. 4. Respiratory infections: Haemophilus influenza and Legionella pneumophilia. 5. GIT infections: shigellosis and non-typhoid salmonella. 6. Systemic salmonella infections: ampicillin or chloramphenicol resistant. Page 10