Chapter 42 Synthesized Antimicrobial Agents Department of pharmacology Liu xiaokang( 刘小康） 2010,3

Quinolones Common properties Structure:

Antimicrobial activity: Quinolones are bactericidal agents. (1) First generation: narrow spectrum (some G-), rapid resistance emergence. e.g. nalidixic acid; (2) second generation: better activety against G – than 1 st Generation, still many advers effects. e.g. pipemidic acid.

(3) third generation: Fluoroquinolones, include norfloxacin, ofloxacin, ciprofloxacin, levofloxacin, lomefloxacin, fleroxacin, sparfloxacin etc. Broad spectrum, Acting on most enterobacter spp. G – bacilli, G + coccus, chlamydia, mycoplasma, legionella and Mycobacterium tuberculosis. (4) Fourth generation: similar antibacterial spectrum with third generation, but greater activity on some anaerobe, G + bacteria and Pseud. aeruginosa.

Mechanism of action: (1) Inhibit the A subunits of DNA gyrase, a type II topoisomerase.

(2) Inhibit topoisomerase IV. Hence inhibits relaxation of supercoiled (packed) DNA needed for DNA replication, increases double-strand DNA breakage. Inhibits two major topisomerases -- specific one favored depends on organism, G – topoisomerase II and G + topoisomerase IV.

(3) The properties of DNA gyrase : (a)Nicks & seals DNA during replication; (b) Needed for DNA to uncoil and recoil; (c) mammalian cells do not have DNA gyrase, but do have stimilar type II DNA topoisomerase. Requires 100 to 1000 μg/mL drug to inhibit these enzymes; (d) Has two A subunits and two B subunits; (e) "A- subunits" are responsible for cutting the DNA. Fluoroquinolones inhhibit this step; (f) Requires 0.1 to 10 μg/ml to inhibit A subunit; (g) Gyrase required for plasmid replication, Fluoroquinolones may reduce plasmid mediated resistance in other drugs.

Pharmacokinetics: Well absorbed, Food not impair absorption; wide distribution, in some tissues concentration is higher than plasma. Pefloxacin, ofloxacin and ciprofloxacin can reach effective concentration in CSF. Renal excretion is primary route for most fluoroquinolones.

Clinical uses: (1) Urinary tract infections, caused by enterococcus, Pseud. aeruginosa and enterobacteriaceae. (2) Intestinal tract infections: G – bacteria, include campylobacter, shigella and salmonella. (3) Respiratory tract infections: e.g. Strep. pneumoniae, hemophilus influenzae and Moraxella infections; (4) Bone and joint infections; (5) Skin and soft tissue infection. (6) Pefloxacin can be used for meningitis or sepsis caused by klebsiella, enterobacter and serratia.

Adverse reactions: (1) Gastrointestinal tract: nausea or vomiting abdominal or stomach pain or discomfort. (2) CNS: Dizziness or lightheadedness, headache, nervousness, drowsiness, insomnia. Rarely incidence: Seizures, CNS Stimulation -- acute psychosis, agitation, confusion, hallucinations, tremors (incidence rare). (3) Allergies: Hypersensitivity can occur rarely, such as angioneuroedema, skin rash and itching, or shortness of breath and photosensitive dermatitis. (5) others: hepatic function impairment, interstitial nephritis.

Contraindication: All quinolones contraindicated in patients with history of convulsions.

Resistance: (1) alterations in DNA gyrase and topoisomerase IV; Mutation of gyrA gene that encode the A subunit confer the resistance, causing reduction of gyrase affinity for drug. (2) Decreased intracellular accumulation of the drug due to modifications of membrane proteins.

Drug interaction: Fluroquinolones interfere with hepatic biotransformation -- may cause toxicity due to excess of the following: theophylline and warfarin.

Properties and clinical uses of commonly used quinolones Norfloxacin: Act on G – bacteria, F=35%- 45%, original form excreted in urine about 45%-60%. Used for treatment of gastrointestinal and urinary tract infections.

Ciprofloxacin: Broad antibacterial spectrum. Higher activity on G + bacteria than norfloxacin. F=38%-60%. Mainly used for urinary tract, gastrointestinal tract, respiratory tract, bone, joint, abdominal cavity, skin and soft tissue infections.

Ofloxacin: Well absorbed, F=80%-90%. Wide distribution, in prostate gland, lung, bone and sputum can reach effective concentration. Used for urinary tract, gastrointestinal tract, respiratory tract, bone, joint, abdominal cavity, skin, soft tissue, biliary tract, eye and ear, nose, throat infections. Also used for the treatment of tuberculosis and mycoplasma infection. Relatively less adverse reactions.

Levofloxacin: The potency is 2 fold of ofloxacin. Antibacterial spectrum and activity are the same with ofloxacin, but less adverse effects.

Lomefloxacin: Well absorbed, F=90%- 98%, t 1/2 =7 h. It can also kill quiescent cell. It also effects on mycoplasma, chlamydia and mycobacterium tuberculosis infection. Special adverse reaction is the photosensitivity.

Sparfloxacin: t 1/2 =17.6 h. wide distributed, similar clinical use with ofloxacin.

Sulfonamides

Common properties of sulfonamides Structure: PABA Sulfonamide

Antibacterial activity: Broad spectrum, bacteriostatic agent. Include G + and G – bacteria, chlamydia, actinomyces etc.

Mechanism of action:

Sulfonamides: (1) Compete with PABA to inhibit synthesis of dihydrofolic acid (DHF). Enzyme: Dihydropteroate synthase (2) False synthesis: Some sulfonamides can be incorporated into DHF-like compound; DHF-like compound can inhibit dihydrofolic acid reductase (DHFR)

Mechanism of Trimethoprim (TMP)

(1) Trimethoprim (TMP) strong inhibitor of DHFR (2) Thus, blocks conversion & recycling of DHF --> THF --> DHF --> THF -->... Trimethoprim affinity for DHFR of bacteria is 50,000 to 60,000 times greater than for that of vertebrates. Therefore, bacterial DHFR is inhibited at concentrations that have essentially no effect on vertebrate DHF.

Mechanisms of Combination: (1) Sequential blockade strong and synergistic; (2) Bactericidal for some bacteria; (3) Blocks synthesis of DHF (sulfa); (4) Thus, blocks conversion & recycling of DHF --> THF --> DHF --> THF -->...

Selectivity: (1) Sulfonamide: (a)Bacteria must make own DHF; (b) Vertebrates require as vitamin (2) Trimethoprim: (a) Different binding affinity for DHFR of vertebrates vs microorganisms; (b) Table compares affinity for mammalian, protozoan, and bacterial enzyme

Concentration Required to Inhibit DHFR by 50% DRUGRAT LIVERE. coliP. berghei IC50 (nM) Pyrimethamine7002,500~0.5 Trimethoprim260, GG 8th, p985. Original data from Ferone, Burchall & Hitchings, 1969

Pharmacokinetics: Most sulfonamides absorbed and excreted rapidly, widely distributed. Metabolized in liver by acetylase and eliminated primarily by glomerular filtration.

Clinical uses: (1) Systemic infection. e.g. meningitis caused by neisseria meningitides. (2) Intestinal tract infection. (3) Burn and hurt infection.

Adverse reactions: (1) Renal crystalluria: (2) Allergies: Immunogenic as haptens. fever, itching, skin rash. Stevens-Johnson syndrome is less frequent (arthralgia and myalgia, redness to blistering of skin, weakness). (3) Blood dyscrasias: Acute hemolytic anemia, Sometimes hypersensitivity or G-6-P dehydrogenase deficiency in RBCs; Agranulocytosis; thrombocytopenia. (4) kernicterus. (5) Hepatic injury.

Resistance: (1) Plasmid-borne and mutations, Change in organism's DHPt synthase to decrease affinity for sulfa (2) Increased PABA -- Some resistant staphylococci produce 70 times as much as susceptible parent strains. (3) Decreased permeability.

Properties and Usages of commonly used sulfa

(1) Systemic using sulfa: Sulfafurazole (SIZ) and sulfadimidine (SM2): Short acting sulfonamides. Sulfadiazine (SD) and sulfamethoxazole (SMZ): Median acting sulfonamides. SD has the best distribution in CSF, 50%~80% of plasma concentration. SMZ has less concentration in CSF, but more solubility. Sulfamonomethoxine (SMM): Long acting sulfonamides.

(2) Topical using sulfa: Sulfasalazine, mafenide (SML), sulfadiazine silver, sulfacetamide.

(3) Combination of TMP- Sulfamethoxazole: co-trimoxazole. Clinical use: (a) Urinary tract infections; (b) Salmonella typhi and salmonella paratyphi infection, and gastrointestinal infections. (c) Bacterial respiratory tract infections, e.g., acute exacerbations of chronic bronchitis; (d) Acute otitis media in children.

Other synthetic antimicrobial agents Trimethoprim (TMP) Antimicrobial activity: It has similar antimicrobial spectrum with sulfonamides, but efficacy is 20~100 fold of sulfa. Mechanisms: see sulfonamides Clinical uses: often combination with sulfamethoxazole (SMZ), similar indications with sulfonamides. (The end)