Lecture 4: Inhibitors of Nucleic Acid Synthesis PDF

INHIBITORS OF NUCLEIC ACID SYNTHESIS Nucleic acids are biopolymers made up of monomers called nucleotides Nucleic acids, including Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA), store genetic information for living organisms. The production and regulation of these biological mac...

INHIBITORS OF NUCLEIC ACID SYNTHESIS Nucleic acids are biopolymers made up of monomers called nucleotides Nucleic acids, including Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA), store genetic information for living organisms. The production and regulation of these biological macromolecules are essential for survival and replication of organisms. Therefore, enzymes involved in these processes are attractive therapeutic targets for a variety of diseases ELONGATION Primase binds to the first priming sequence on the leading strand template and synthesizes a short RNA primer that is complementary to the DNA template DNA polymerase uses the primer to initiate DNA synthesis by adding deoxyribonucleotides to the 3’ end. The leading strand requires only one priming event because DNA synthesis is continuous thereafter in the 3’5’ direction ENZYME FUNCTION HELICASE UNZIPS THE DOUBLE STRAND STRUCTURE OF THE DNA DNA POLYMERASE MATCHES AND ADDS NEW NUCLEOTIDES TO FORM DAUGHTER DNA STRAND PRIMASE SYNTHESIS A SHORT STRAND OF RNA CALLED PRIMER DNA LIGASE JOINS TOGETHER THE OKAZAKI FRAGMENTS OF THE LAGGING STRAND TOPOISOMERASE RELIEVES THE STRESS GENERATED BY UNZIPPING OF DNA EXONUCLEASE REMOVES THE SHORT SEGMENTS OF PRIMERS AND IS REPLACED BY DNA POLYMERASE QUINOLONES Nitroimidazoles and Nitrofurans E.g Metronidazole and Nitrofurantoin Cause DNA strand breakage but by a direct chemical action rather than inhibition of enzymes The nitro group of metronidazole is converted to nitronate radical by low redox potentials within the cells The activated metronidazole then attacks the DNA producing strand breakage SPECTRUM OF ACTIVITY Metronidazole is active only against anaerobic organisms Anaerobic Gram-negative bacilli: Bacteroides fragilis, Bacteroides species, Fusobacterium spp., Porphyromonas spp., Prevotella spp. Anaerobic Gram-positive bacilli: Clostridium spp. Anaerobic gram positive cocci: Peptostreptococcus species, Veillonella species It is also an antiprotozoan Protozoa: Blastocystis hominis, Entamoeba histolytica, Giardia lamblia, Trichomonas vaginalis Rifamycins The drug rifampicin is a semisynthetic member of the rifamycin family and functions by blocking RNA polymerase activity in bacteria which is responsible for transcription of DNA to RNA The RNA polymerase enzymes in bacteria are structurally different from those in eukaryotes, providing for selective toxicity against bacterial cells. It is used for the treatment of a variety of infections, but its primary use, often in a cocktail with other antibacterial drugs, is against mycobacteria that cause tuberculosis. 
INHIBITORS OF METABOLIC PATHWAYS

FOLATE METABOLISM Folic acid is an important co-factor in all living cells In its reduced form (Tetrahydrofolate THF) functions as a carrier of single carbon fragments which are used in the synthesis of adenine, guanine, thymine and methionine Bacteria, protozoa and mammalian cells all possess Dihydrofolate receptor(DHFR) but there are sufficient differences in the enzyme structure for inhibitors such as trimethoprim to inhibit bacterial enzymes selectively Another fundamental difference between folate utilization in microbial and mammalian cells is that bacteria and protozoa are unable to take up exogenous folate and must synthesize Mammalian cells do not make their own DHF , instead they take it up from dietary nutrients and convert it to THF using DHFR SULPHONAMIDES AND SULFONES E.g Sulphamethoxazole and Dapsone They are structural analogues of PABA They competitively inhibit the incorporation of PABA into dihydropteroic acid which inhibit subsequent metabolism This mechanism of action provides bacteriostatic inhibition of growth against a broad spectrum of gram-positive and gram- negative pathogens TRIMETHOPRIM Synthetic antimicrobial compound that serves as an antimetabolite within the same folic acid synthesis pathway as sulphonamides. However, trimethoprim is a selective inhibitor of bacterial dihydrofolate reductase Trimethoprim is used in combination with the sulfa drug sulfamethoxazole to treat urinary tract infections, ear infections, and bronchitis. As discussed, the combination of trimethoprim and sulfamethoxazole is an example of antibacterial synergy. When used alone, each antimetabolite only decreases production of folic acid to a level where bacteriostatic inhibition of growth occurs. However, when used in combination, inhibition of both steps in the metabolic pathway decreases folic acid synthesis to a level that is lethal to the bacterial cell.  ANTIFUNGAL AGENTS Fungi are eukaryotic (have a nucleus and well defined nuclear membrane and chromosomes) Heterotrophic organisms that live as saprobes(feeding on decay metals) or as parasites Complex organisms compared to bacteria Have a rigid cell wall composed of chitin (bacterial cell wall is composed of peptidoglycan) Fungal cell membrane contains ergosterol, human cell membrane is composed of cholesterol Antibacterial agents are not effective against fungi Fungal infections are also called mycoses TYPES OF FUNGI Yeasts: reproduce by budding; e.g Cryptococcus neoformans which causes cryptococcal meningitis Yeast like fungi: grows partly as yeast and partly as filaments called hyphae e.g Candida albicans causes oral, vaginal and systemic candidiasis Dimorphic fungi:which grows as mould or as yeast e.g Histoplasma capsulatum which can cause histoplasmosis Moulds: Filamentous fungi which reproduce by forming spores e.g Dermatophytes (Tricophyton spp, Microsporum spp, Epidermophyton spp) which cause hair, skin and nail infections such as ringworm, Aspergillus spp NYSTATIN Isolated from Streptomyces noursei Polyene macrolide similar to amphotericin B in structure and action Used for superficial candidiasis of the mouth, skin, vagina and intestine More toxic than Amphotericin B when used parenterally ALLYLAMINE E.g Terbinafine, butenafine, naftifine Inhibit the fungal enzyme Squalene epoxidase which converts squalene to lanosterol Leads to reduction in lanosterol production that reduces ergosterol production which affects cell membrane function and integrity Fungicidal The most commonly used allylamine is terbinafine (marketed under the brand name Lamisil), which is used topically for the treatment of dermatophytic skin infections like athlete’s foot, ringworm, and jock itch. Oral treatment with terbinafine is also used for the treatment of fingernail and toenail fungus, but it can be associated with the rare side effect of hepatotoxicity. Echinocandins The echinocandins, including caspofungin, are a group of naturally produced antifungal compounds Capsofungin is derived from fermentation by product of Glarea lozoyensis Blocks the synthesis of β(1→3) glucan found in fungal cell walls but not found in human cells. This drug class has the nickname “penicillin for fungi.” Caspofungin is used for the treatment of aspergillosis as well as systemic yeast infections. Others are Micafungin and andulafungin which have similar efficacy against Candida species  Other topical agents Tolnaftate which is effective in treatment of most cutaneous mycoses caused by Trichophyton rubrum, T. tonsurans, ,Mycosporum gypseum and M. canis but it is ineffective against Candida Undecylenic acid used in treatment of various dermatomycoses especially tinea pedis, it is primarily fungistatic but fungicidal activity may be observed with long exposure to high concentrations of the agent Benzoic acid and salicylic acid; components of whitfield’s ointment. It combines the fungistatic action of benzoate with keratolytic action of salicylate. Used for the treatment of tinea pedis