Metabolic & Nucleic Acid Inhibitors Reading assignments: Katzung’s Basic & Clinical Pharmacology, 13th Edi ,Ch-46,p807-814;

D. Inhibitors of folate-dependent pathways LEARNING OBJECTIVES D. Inhibitors of folate-dependent pathways Understand the production and use of folate derivatives in bacterial systems 1. Sulfonamides Know the mechanism of action of sulfonamides Understand the concept of "antimetabolite" Appreciate the role of pharmacokinetics in the action and uses of sulfonamides Know the pharmacokinetic and pharmacodynamic differences among various sulfonamides Know the adverse effects of sulfonamides 2. Trimethoprim Know the mechanism of action of trimethoprim Understand the rationale of combined sulfonamide-trimethoprim chemotherapy Know the clinical uses and adverse effects associated with trimethoprim E. DNA gyrase inhibitors Understand the function of DNA gyrases, and the effects of their inhibition Know the clinical uses of quinolones and fluoroquinolones Know the adverse effects and potential drug-drug interaction for quinolones F. Nitroimidazoles Understand the mechanism of action of Metronidazole & tinidazole Know the clinical uses Know the adverse effects and potential drug-drug interaction G. Urinary tract antiseptics Understand the role of pharmacokinetics in the treatment of urinary tract infections

D. Inhibitors of folate-dependent pathways 1. Sulfonamides SULFISOXAZOLE SULFAMETHOXAZOLE (1) SULFASALAZINE (SALICYLAZOSULFAPYRIDINE)(2) SODIUM SULFACETAMIDE SILVER SULFADIAZINE (3) CO-TRIMOXAZOLE (SULFAMETHOXAZOLE/TRIMETHOPRIM) (7) 2. Dihydrofolate reductase inhibitors TRIMETHOPRIM (bacteria) (4) PYRIMETHAMINE (protozoa) (5) METHOTREXATE (mammalian) (6)

Anti-Metabolites: Anti-Folates Two Classes— Inhibitors of folate synthesis p-Aminobenzoic acid analogs (PABA) Inhibitors of folate use dihydrofolate reductase inhibitors Trimethoprim – bacterial Pyrimethamine – protozoa Methotrexate - mammalian #Folic acid requirement is for1. methylation required for the conversion of uracil to thymine 2. adenine & 3.guanine synthesis (indirect effect on NA synthesis) #Bacteria synthesizes FA from junk through Dihydropteroate synthase enzyme which is absent in human.Hence sufonamide donot cause toxicity that stems from inhibition of Folic acid in human beings eg.bone marrow depression etc #Human has DHFR like bacteria, protozoa therefore drugs that are designed to inhibit DHFR in bacteria or protozoa may cross inhibit DHFR in human ,may cause BMS etc. 1.A 25-year-old female with a sinus infection caused by Haemophilus influenzae is treated with trimethoprim-sulfamethoxazole.What is the Mechanism of action of those drugs?

Sulfonamides Mechanism PABA analogs Enter into a normal metabolic pathway, but then block that pathway Competitive inhibitor of dihydropteroate synthase Bacteriostatic Pharmacokinetics Oral, some topical (burns), rarely IV Well absorbed from GI, high PPB, well distributed including to CNS Variable metabolism, depending on drug and patient Acetylation yields inactive metabolite (less water soluble)→ Crystalluria Excreted in urine (90% by glomerular filtration) 10-20x blood concentration in urine

Sulfonamides: Clinical Aspects Clinical Use-- Topical for burns--Silver sulfadiazine Ophthalmic preparations – Sodium sulfacetamide Urinary Tract Infections-- Uncomplicated, untreated, acute Ulcerative colitis-- Sulfasalazine, Mesalamine Not absorbed-- split by gut bacteria reductase to release 5-aminosalicylate Sulfonamides now are seldom used as single agents-- combine with trimethoprim Adverse Effects-- Allergic reactions-- fever, rash, etc. Up to 5% incidence Cross-reactivity with other sulfonamides-- carbonic anhydrase inhibitors, thiazides, furosemide, sulfonylurea hypoglycemics Crystalluria Sulfonamide alone is almost never used as an antibiotic 1.A 26-year-old woman presents to the emergency department with a third-degree burn on her right forearm. She states that she accidentally spilled boiling water on her arm. Which of the following topical medications would be the most appropriate treatment of her condition? Betamethasone cream Calcipotriene ointment Erythromycin solution Metronidazole cream Silver sulfadiazine cream

Sulfonamides: Clinical Aspects Adverse Effects– Stevens-Johnson syndrome—(Type IV) Fever, malaise, . . . Rare, but can be fatal Hematopoietic effects Hemolytic anemias-- G6PDH deficiency Drug Interactions Kernicterus in newborns Resistance-- Mutations causing overproduction of PABA Loss of permeability New form of dihydropteroate synthetase-discriminates between PABA and sulfonamide Sulfonamide-Sulfur(S)+Amide(PABA analogue)=hence so much allergic Therapeutic Rationale for Kernicterus : Treatment of neonatal meningitis (m/c organism E.Coli or St.Agalactae) 1.A jaundiced one-day-old premature infant with an elevated free bilirubin is seen in the premature-baby nursery. The mother received an antibiotic combination preparation containing sulfamethizole for a urinary tract infection (UTI) one week before delivery. You suspect that the infant’s findings are caused by the sulfonamide because of the following mechanism: a. Enhanced synthesis of bilirubin b. Competition between the sulfonamide and bilirubin for binding sites on albumin c. Inhibition of bilirubin degradation d. Inhibition of urinary excretion of bilirubin The answer is b. Sulfonamides should not be used in pregnant women who are at term because of their ability to cross the placenta and enter the fetus in concentrations sufficient to produce toxic effects. Sulfonamides should also not be given to neonates, especially premature infants, because they compete with bilirubin for serum albumin binding, resulting in increased levels of free bilirubin, which cause kernicterus.

Dihydrofolate Reductase Inhibitors Trimethoprim-- blocks bacterial enzyme Pyrimethamine-- protozoan enzyme Methotrexate-- mammalian enzyme Trimethoprim-- Readily absorbed from GI Wide distribution, including CNS Excreted in urine Can be used alone for UTI, but usually combined with a sulfonamide

Trimethoprim-Sulfamethoxazole (Co-trimoxazole) Combination is often bactericidal DOC in Nocardia Mycobacteria Gm-ve Infections (E.Coli, Salmonella,Shihella) Gm+ve Infections ( Staph, Strepto, H. Influenzae ) Fungus: Pneumocystis jiroveci pneumonia (PCP)-both for prophylaxis (when CD4 count <200) & treatment in HIV infection. Protozoa : Toxoplasma Gondii (Sulfadiazine + Pyrimethamine) both for prophylaxis (when CD4 count <200) & treatment in HIV infection & in T of STORCH infection. Note: Pneumocystis jiroveci used to be called Pneumocystis carinii (PCP)

Adverse Effects of Trimethoprim Trimethoprim is 10,000x more effective against bacterial DHFR than against the mammalian enzyme, but still may see "anti-folate" effects Megaloblastic anemia, leukopenia, granulocytopenia Treat with folinic acid Co-trimoxazole-- Add typical sulfonamide effects AIDS patients with Pneumocystis pneumonia (PCP) receiving Co-trimoxazole-- Much higher incidence of adverse effects Fever, rashes, leukopenia, diarrhea 1.A newborn boy is diagnosed with a disorder caused by high levels of unconjugated bilirubin. Which of the following drugs administered to the infant’s mother during the last trimester of pregnancy might account for this presentation? Acetaminophen Azithromycin Chloramphenicol Diethylstilbestrol Trimethoprim-sulfamethoxazole

Common ending with ‘floxacin’ E. DNA gyrase inhibitors NALIDIXIC ACID NORFLOXACIN CIPROFLOXACIN LEVOFLOXACIN GATIFLOXACIN SPAROFLOXACIN MOXIFLOXACIN TROVAFLOXACIN Common ending with ‘floxacin’ Flo-fluorine

DNA Gyrase Inhibitors Quinolones Nalidixic Acid-- Fluoroquinolones-- Prototype quinolone antibiotic Inhibits DNA replication [by inhibiting DNA gyrase (Topoisomerase II)& IV] Pharmacokinetics Oral administration Rapidly absorbed, metabolized (Glucuronidated) and excreted in urine Fluoroquinolones-- Norfloxacin Ciprofloxacin Ofloxacin Levofloxacin Gatifloxacin Moxifloxacin Gemifoxacin Fluorinated analogues of nalidixic acid Well absorbed and distributed after oral administration (Iron,Calcium limit absorption ) Only 20% is metabolized (liver) Excreted in urine, blocked by probenecid Effective systemically after oral dose, parenteral forms also available Note: -floxacin ending #Topoisomerase are required for supercoiling (over & under twisting of DNA called +ve & -ve super coiling respectively). All Topoisomerases decreases supercoiling allowing other enzymes to express for replication.Particularly Topoisomerase II helps in –ve supercoinling (opening DNA to help processing of replication & gene expression).All fluroquinolone prevent introduction of negitive supercoiling. #Topoisomerase II keeps the recently separated DNA stands in position (by inhibiting cystochromatids) until G2 phase starts in. 1.A 36-year-old female with a chronic UTI treated with ciprofloxacin is not responsive to the antibiotic. Which of the following agents that she might have been taking for other reasons would decrease the effectiveness of ciprofloxacin? a. An antacid b. An antihistamine c. A nonsteroidal anti-inflammatory d. An anxiolytic e. A multivitamin not containing iron 2.A 39-year-old female with a history of chronic UTI develops a new infection with Escherichia coli that is sensitive to levofloxacin.What is the Mechanism of action of Levofloxacin (or any quinolone)?

Fluoroquinolones Example 1st Generation: norfloxacin; activity against common pathogens that cause urinary tract infections; similar to nalidixic acid Examples of 2nd Generation: ciprofloxacin, ofloxacin; excellent activity against gram-negative bacteria, including gonococcus, Chlamydia, many gram-positive cocci, mycobacteria, and Mycoplasma pneumoniae Examples of 3rd Generation: levofloxacin, gatifloxacin; less activity against gram-negative bacteria but greater activity against some gram-positive cocci, such as S. pneumoniae, entercocci, and MRSA; good for many drug-resistant respiratory tract infections Examples of 4th Generation: moxifloxacin, Gemifloxacin; broadest spectrum fluoroquinolones with good activity against anaerobes

Activity & Clinical Uses -UTIs particularly when resistant to Cotrimoxazole -STDs/PIDs:Chlamydia (Ofloxacin), Gonorrhea (Cipro,Ofloxacin) -Skin, soft tissue & bone infection by gm-ve organisms (all except norfloxacin) -Diarrhea due to Shigella ,Salmonella, E.coli, Campylobacter (any quinolone) -PNSP (Levofloxacin)

Fluoroquinolones Adverse Effects-- Nausea and vomiting Phototoxicity & rashes (other drugs?) All quinolone ↑ QT interval Headaches, dizziness, insomnia Abnormal liver function tests Blocks theophylline clearance-- cannot be co-administered Connective tissue disorders including tendonitis or tendon rapture (in adults), myalgia & leg cramps (in children) Do not use during pregnancy or in children (Fluorine is the culprit ) Resistance-- Altered (mutated) DNA gyrase Especially Pseudomonas, Staph, Serratia No longer recommended for gonococcus because of resistance #Connective tissue disorder as an a/e of FQ occurs due to inhibition of host DNA gyrase responsible for replication in connective tissue 1.Which of the following may cause damage to growing cartilage? a. Fluoroquinolones b. Sulfonamides c. Aminoglycosides d. Cephalosporins e. Tetracyclines The answer is a. Fluoroquinolones are not recommended in patients less than 18 years old. They have a tendency to damage growing cartilage and cause arthropathy. The arthropathy is generally reversible. Tendinitis may occur, and in rare instances in adults, this finding may lead to tendon ruptures.

Discuss about the rationality of long term use of fluoroquinolones in children , adults or in pregnancy.

F. Urinary tract antiseptics NITROFURANTOIN SYSTEMIC AGENTS

Urinary Tract Antiseptics Use systemic agents, which are efficiently cleared in the urine: Penicillins Aminoglycosides Sulfonamides Resistance and re-infection are common May need to suppress bacteria for a long time Common UTI bugs are E.Coli(m/c), staphylococcus saprophyticus ,enterobacter cloacae, proteus mirabilis,klebsiella pneumonae, pseudomonus aeruginosa and serriatia .

Nitrofurantoin Mechanism Unknown, but may involve oxidative stress Bacteriostatic or bactericidal (depends on microbe) Pharmacokinetics-- Rapidly absorbed (oral), metabolized , and excreted in urine (50% as active drug) Even IV nitrofurantoin does not have a systemic effect Clinical Use-- UTI, gram positive or gram negative microbes Most effective if urine pH < 5.5 Adverse Effects-- Anorexia, GI disturbances common, headaches Occasional hemolytic anemia (oxidative) especially if G6PDH deficient, leukopenia, hepatotoxicity In renal insufficiency, see systemic toxicity Resistance-- All Pseudomonas, some Proteus are resistant

Spectrum coverage of Antibiotics covered in a chart.

Antibiotic Sensitivity Overview

Antibiotics of Choice for Various Infections Drug Organism (Disease) Amoxicillin, clarithromycin (omeprazole*) Helicobacter pylori (peptic ulcer) Ampicillin Listeria (meningitis) Ceftriaxone, Cefexime Neisseria gonorrhoeae (gonorrhea) Cephalosporins (third-generation) Haemophilus influenzae (pneumonia, meningitis) Klebsiella (meningitis) Doxycycline Borrelia burgdorferi (Lyme disease) Rickettsiae (Rocky Mountain spotted fever) Erythromycin Legionella (legionnaires’ disease) Fluconazole, miconazole, nystatin Candida (candidiasis) Isoniazid, rifampin, ethambutol, Mycobacterium tuberculosis pyrazinamide (tuberculosis) *Omeprazole is not an antibiotic but is used in combination with antibiotics for treatment of H. pylori.

Antibiotics of Choice for Various Infections Drug Organism (Disease) Macrolides Mycoplasma pneumoniae (atypical pneumonia) Legionella (legionnaires’ disease) Corynebacterium diphtheriae; Chlamydia Metronidazole Trichomonas (trichomoniasis) Penicillin G Neisseria meningitidis (meningitis) Treponema pallidum (syphilis) Infections caused by streptococci, pneumococci, other meningococci, Bacillus anthracis, Clostridium, Bacteroides (except B. fragilis) Fluroquinolones Campylobacter (diarrhea); Shigella Tetracycline Vibrio cholerae (cholera) Other tetracyclines Chlamydia (pneumonia, lymphogranuloma venereum) Trimethoprim-sulfamethoxazole Salmonella; Shigella (diarrhea) Metronidazole or vancomycin (oral) Clostridium difficile (diarrhea) 1. A man is with fever, chills, and a productive cough with sputum Gram's stain reveals numerous gram-positive diplococci. Which of the following antibiotics would be the most appropriate choice for empiric therapy of this man's infection?

Prophylactic Use of Anti-infective Drugs Drug Use Cefazolin Surgical procedures Cefoxitin, Cefotetan Surgical procedures where anaerobic infections are common Ampicillin or penicillin Group B streptococcal infections Trimethoprim-sulfamethoxazole Pneumocystis carinii pneumonia (PCP) UTIs Rifampin Haemophilus influenzae type B Meningococcal infection Chloroquine Malaria Isoniazid Tuberculosis Azithromycin Mycobacterium avium complex (MAC) in patients with AIDS Ampicillin or azithromycin or clindamycin Dental procedures in patients with valve abnormalities Prophylaxis for a patient w/murmur attending a dentist for tooth extraction ? AIDS = Acquired immunodeficiency syndrome; UTI = urinary tract infection.

Pyrimethamine Methotrexate -floxain

A 44-year-old woman recently exposed to influenza experiences a headache, fever, malaise and a cough. These symptoms subside after 5 days but on the 8th day her cough worsen and she again has a fever. Physical examination reveals signs of pneumonia. Which of the following medications would be most appropriate for the treatment of pneumonia in this patient? Amantadine Levofloxacin Palivizumab Ribavirin Sulfamethoxazole Answer: B Levofloxacin is a respiratory fluoroquinolone

PowerPoint Slides Several of the PowerPoint slides are Copyright © 2002-04, the  American Society for Pharmacology and Experimental Therapeutics (ASPET). All rights reserved. Some of slides in this session are from the above mentioned format and are free for use by members of ASPET.  Some others are from various sources like text book, recommended books, slides of Dr. S. Akbar (ex. professor, Pharmacology ,MUA). Core concepts of various USMLE High yield review series like Kaplan ,BRS etc. are thoroughly explored & integrated whenever necessary