книги студ / Color Atlas of Pharmacology

Inhibitors of Tetrahydrofolate Synthesis

Tetrahydrofolic acid (THF) is a co-en- zyme in the synthesis of purine bases and thymidine. These are constituents of DNA and RNA and required for cell growth and replication. Lack of THF leads to inhibition of cell proliferation. Formation of THF from dihydrofolate (DHF) is catalyzed by the enzyme dihydrofolate reductase. DHF is made from folic acid, a vitamin that cannot be synthesized in the body, but must be taken up from exogenous sources. Most bacteria do not have a requirement for folate, because they are capable of synthesizing folate, more precisely DHF, from precursors. Selective interference with bacterial biosynthesis of THF can be achieved with sulfonamides and trimethoprim.

Sulfonamides structurally resemble p-aminobenzoic acid (PABA), a precursor in bacterial DHF synthesis. As false substrates, sulfonamides competitively inhibit utilization of PABA, hence DHF synthesis. Because most bacteria cannot take up exogenous folate, they are depleted of DHF. Sulfonamides thus possess bacteriostatic activity against a broad spectrum of pathogens. Sulfonamides are produced by chemical synthesis. The basic structure is shown in

(A). Residue R determines the pharmacokinetic properties of a given sulfonamide. Most sulfonamides are well absorbed via the enteral route. They are metabolized to varying degrees and eliminated through the kidney. Rates of elimination, hence duration of effect, may vary widely. Some members are poorly absorbed from the gut and are thus suitable for the treatment of bacterial bowel infections. Adverse effects may include, among others, allergic reactions, sometimes with severe skin damage, displacement of other plasma protein-bound drugs or bilirubin in neonates (danger of kernicterus, hence contraindication for the last weeks of gestation and in the neonate). Because of the frequent emergence of resistant bacteria, sulfonamides are now rarely used.

Introduced in 1935, they were the first broad-spectrum chemotherapeutics.

Trimethoprim inhibits bacterial DHF reductase, the human enzyme being significantly less sensitive than the bacterial one (rarely bone marrow depression). A 2,4-diaminopyrimidine, trimethoprim, has bacteriostatic activity against a broad spectrum of pathogens. It is used mostly as a component of cotrimoxazole.

Co-trimoxazole is a combination of trimethoprim and the sulfonamide sul- famethoxazole. Since THF synthesis is inhibited at two successive steps, the antibacterial effect of co-trimoxazole is better than that of the individual components. Resistant pathogens are infrequent; a bactericidal effect may occur. Adverse effects correspond to those of the components.

Although initially developed as an antirheumatic agent (p. 320), sulfasala- zine (salazosulfapyridine) is used mainly in the treatment of inflammatory bowel disease (ulcerative colitis and terminal ileitis or Crohn’s disease). Gut bacteria split this compound into the sulfonamide sulfapyridine and mesala- mine (5-aminosalicylic acid). The latter is probably the anti-inflammatory agent (inhibition of synthesis of chemotactic signals for granulocytes, and of H2O2 formation in mucosa), but must be present on the gut mucosa in high concentrations. Coupling to the sulfonamide prevents premature absorption in upper small bowel segments. The cleaved-off sulfonamide can be absorbed and may produce typical adverse effects (see above).

Dapsone (p. 280) has several therapeutic uses: besides treatment of leprosy, it is used for prevention/prophylaxis of malaria, toxoplasmosis, and actinomycosis.

Inhibitors of DNA Function

Deoxyribonucleic acid (DNA) serves as a template for the synthesis of nucleic acids. Ribonucleic acid (RNA) executes protein synthesis and thus permits cell growth. Synthesis of new DNA is a prerequisite for cell division. Substances that inhibit reading of genetic information at the DNA template damage the regulatory center of cell metabolism. The substances listed below are useful as antibacterial drugs because they do not affect human cells.

Gyrase inhibitors. The enzyme gyrase (topoisomerase II) permits the orderly accommodation of a ~1000 µmlong bacterial chromosome in a bacterial cell of ~1 µm. Within the chromosomal strand, double-stranded DNA has a double helical configuration. The former, in turn, is arranged in loops that are shortened by supercoiling. The gyrase catalyzes this operation, as illustrated, by opening, underwinding, and closing the DNA double strand such that the full loop need not be rotated.

Derivatives of 4-quinolone-3-car- boxylic acid (green portion of ofloxacin formula) are inhibitors of bacterial gyrases. They appear to prevent specifically the resealing of opened strands and thereby act bactericidally. These agents are absorbed after oral ingestion. The older drug, nalidixic acid, affects exclusively gram-negative bacteria and attains effective concentrations only in urine; it is used as a urinary tract antiseptic. Norfloxacin has a broader spectrum. Ofloxacin, ciprofloxacin, and enoxacin, and others, also yield systemically effective concentrations and are used for infections of internal organs.

Besides gastrointestinal problems and allergy, adverse effects particularly involve the CNS (confusion, hallucinations, seizures). Since they can damage epiphyseal chondrocytes and joint cartilages in laboratory animals, gyrase inhibitors should not be used during pregnancy, lactation, and periods of growth.

Azomycin (nitroimidazole) derivatives, such as metronidazole, damage

DNA by complex formation or strand breakage. This occurs in obligate anaerobes, i.e., bacteria growing under O2 exclusion. Under these conditions, conversion to reactive metabolites that attack DNA takes place (e.g., the hydroxylamine shown). The effect is bactericidal. A similar mechanism is involved in the antiprotozoal action on Trichomonas vaginalis (causative agent of vaginitis and urethritis) and Entamoeba histolytica

(causative agent of large bowel inflammation, amebic dysentery, and hepatic abscesses). Metronidazole is well absorbed via the enteral route; it is also given i.v. or topically (vaginal insert). Because metronidazole is considered potentially mutagenic, carcinogenic, and teratogenic in the human, it should not be used longer than 10 d, if possible, and be avoided during pregnancy and lactation. Timidazole may be considered equivalent to metronidazole.

Rifampin inhibits the bacterial enzyme that catalyzes DNA template-di- rected RNA transcription, i.e., DNA-de- pendent RNA polymerase. Rifampin acts bactericidally against mycobacteria (M. tuberculosis, M. leprae), as well as many gram-positive and gram-negative bacteria. It is well absorbed after oral ingestion. Because resistance may develop with frequent usage, it is restricted to the treatment of tuberculosis and leprosy (p. 280).

Rifampin is contraindicated in the first trimester of gestation and during lactation.

Rifabutin resembles rifampin but may be effective in infections resistant to the latter.

Inhibitors of Protein Synthesis

Protein synthesis means translation into a peptide chain of a genetic message first copied (transcribed) into m- RNA (p. 274). Amino acid (AA) assembly occurs at the ribosome. Delivery of amino acids to m-RNA involves different transfer RNA molecules (t-RNA), each of which binds a specific AA. Each t-RNA bears an “anticodon” nucleobase triplet that is complementary to a particular m-RNA coding unit (codon, consisting of 3 nucleobases.

Incorporation of an AA normally involves the following steps (A):

1.The ribosome “focuses” two codons on m-RNA; one (at the left) has bound its t-RNA-AA complex, the AA having already been added to the peptide chain; the other (at the right) is ready to receive the next t-RNA-AA complex.

2.After the latter attaches, the AAs of the two adjacent complexes are linked by the action of the enzyme peptide synthetase (peptidyltransferase). Concurrently, AA and t-RNA of the left complex disengage.

3.The left t-RNA dissociates from m-RNA. The ribosome can advance along the m-RNA strand and focus on the next codon.

4.Consequently, the right t-RNA- AA complex shifts to the left, allowing the next complex to be bound at the right.

These individual steps are susceptible to inhibition by antibiotics of differ-

ent groups. The examples shown originate primarily from Streptomyces bacteria, some of the aminoglycosides also being derived from Micromonospora bacteria.

1a. Tetracyclines inhibit the binding of t-RNA-AA complexes. Their action is bacteriostatic and affects a broad spectrum of pathogens.

1b. Aminoglycosides induce the binding of “wrong” t-RNA-AA complexes, resulting in synthesis of false proteins. Aminoglycosides are bactericidal. Their activity spectrum encompasses

mainly gram-negative organisms. Streptomycin and kanamycin are used predominantly in the treatment of tuberculosis.

Note on spelling: -mycin designates origin from Streptomyces species; -mi- cin (e.g., gentamicin) from Micromonospora species.

2.Chloramphenicol inhibits peptide synthetase. It has bacteriostatic activity against a broad spectrum of pathogens. The chemically simple molecule is now produced synthetically.

3.Erythromycin suppresses ad-

vancement of the ribosome. Its action is predominantly bacteriostatic and directed against gram-positve organisms. For oral administration, the acid-labile base (E) is dispensed as a salt (E. stearate) or an ester (e.g., E. succinate). Erythromycin is well tolerated. It is a suitable substitute in penicillin allergy or resistance. Azithromycin, clarithromy- cin, and roxithromycin are derivatives with greater acid stability and better bioavailability. The compounds mentioned are the most important members of the macrolide antibiotic group, which includes josamycin and spiramycin. An unrelated action of erythromycin is its mimicry of the gastrointestinal hormone motiline (! interprandial bowel motility).

Clindamycin has antibacterial activity similar to that of erythromycin. It exerts a bacteriostatic effect mainly on gram-positive aerobic, as well as on anaerobic pathogens. Clindamycin is a semisynthetic chloro analogue of lincomycin, which derives from a Streptomyces species. Taken orally, clindamycin is better absorbed than lincomycin, has greater antibacterial efficacy and is thus preferred. Both penetrate well into bone tissue.

Tetracyclines are absorbed from the gastrointestinal tract to differing degrees, depending on the substance, absorption being nearly complete for doxycycline and minocycline. Intrave- nous injection is rarely needed (rolite- tracycline is available only for i.v. administration). The most common unwanted effect is gastrointestinal upset

(nausea, vomiting, diarrhea, etc.) due to

(1) a direct mucosal irritant action of these substances and (2) damage to the natural bacterial gut flora (broad-spec- trum antibiotics) allowing colonization by pathogenic organisms, including Candida fungi. Concurrent ingestion of antacids or milk would, however, be inappropriate because tetracyclines form insoluble complexes with plurivalent cations (e.g., Ca2+, Mg2+, Al3+, Fe2+/3+) resulting in their inactivation; that is, absorbability, antibacterial activity, and local irritant action are abolished. The ability to chelate Ca2+ accounts for the propensity of tetracyclines to accumulate in growing teeth and bones. As a result, there occurs an irreversible yellowbrown discoloration of teeth and a reversible inhibition of bone growth. Because of these adverse effects, tetracycline should not be given after the second month of pregnancy and not prescribed to children aged 8 y and under. Other adverse effects are increased photosen- sitivity of the skin and hepatic damage, mainly after i.v. administration.

The broad-spectrum antibiotic chloramphenicol is completely absorbed after oral ingestion. It undergoes even distribution in the body and readily crosses diffusion barriers such as the blood-brain barrier. Despite these advantageous properties, use of chloramphenicol is rarely indicated (e.g., in CNS infections) because of the danger of bone marrow damage. Two types of bone marrow depression can occur: (1) a dose-dependent, toxic, reversible form manifested during therapy and, (2) a frequently fatal form that may occur after a latency of weeks and is not dose dependent. Due to high tissue penetrability, the danger of bone marrow de-

pression must also be taken into account after local use (e.g., eye drops).

Aminoglycoside antibiotics con- sist of glycoside-linked amino-sugars (cf. gentamicin C1α, a constituent of the gentamicin mixture). They contain numerous hydroxyl groups and amino groups that can bind protons. Hence, these compounds are highly polar, poorly membrane permeable, and not absorbed enterally. Neomycin and paro- momycin are given orally to eradicate intestinal bacteria (prior to bowel surgery or for reducing NH3 formation by gut bacteria in hepatic coma). Aminoglycosides for the treatment of serious infections must be injected (e.g., gen- tamicin, tobramycin, amikacin, netilmicin, sisomycin). In addition, local inlays of a gentamicin-releasing carrier can be used in infections of bone or soft tissues. Aminoglycosides gain access to the bacterial interior by the use of bacterial transport systems. In the kidney, they enter the cells of the proximal tubules via an uptake system for oligopeptides. Tubular cells are susceptible to damage (nephrotoxicity, mostly reversible). In the inner ear, sensory cells of the vestibular apparatus and Corti’s organ may be injured (ototoxicity, in part irreversible).

Tetracyclines

Inactivation by chelation of Ca2+, Al3+ etc.

Irritation of mucous membranes

Absorption

Antibacterial effect on gut flora

Chelation

Chloramphenicol

Advantage:

good penetration through barriers

Disadvantage: bone marrow toxicity

e.g.,

neomycin

Gentamicin C1a

High hydrophilicity no passive diffusion

through membranes H+ +

H+ H

Cochlear and vestibular ototoxicity

Drugs for Treating Mycobacterial Infections

Mycobacteria are responsible for two diseases: tuberculosis, mostly caused by

M. tuberculosis, and leprosy due to M. leprae. The therapeutic principle applicable to both is combined treatment with two or more drugs. Combination therapy prevents the emergence of resistant mycobacteria. Because the antibacterial effects of the individual substances are additive, correspondingly smaller doses are sufficient. Therefore, the risk of individual adverse effects is lowered. Most drugs are active against only one of the two diseases.

Antitubercular Drugs (1)

contraceptives). Concerning rifabutin see p. 274.

Ethambutol. The cause of its specific antitubercular action is unknown. Ethambutol is given orally. It is generally well tolerated, but may cause dosedependent damage to the optic nerve with disturbances of vision (red/green blindness, visual field defects).

Pyrazinamide exerts a bactericidal action by an unknown mechanism. It is given orally. Pyrazinamide may impair liver function; hyperuricemia results from inhibition of renal urate elimination.

Streptomycin must be given i.v. (pp. 278ff) like other aminoglycoside antibiotics. It damages the inner ear and the labyrinth. Its nephrotoxicity is comparatively minor.