Living Textbook MC610

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Antimycobacterial Agents

Mycobacterium are characterized by a complex cell wall that is highly hydrophobic due to a high lipid content, with a backbone made of Mycolic acid, D-Arabinose and Peptidoglycan. It shows low permeability, preventing many antibiotics from being effective. It is identified as an acid fast bacilli, where the cell wall can be stained but not destained by acids.

Mycobacterium causes two major diseases: Leprosy , a disease mentioned in the bible that is more common in tropical countries, and inflicts 10-20 million people worldwide and Tuberculosis, an infamous disease that has claimed the life of many famous people including Chopin, George Orwell, Anton Chekhov, John Keats, Vivien Leigh and Camille. It is estimated that one third to one half of the world population is infected by Mycobacterium tuberculosis and that 6% of all deaths worldwide is due to this disease, making it the most deadly infectious bacterial disease. In the US, TB was on the decline until 1984, where it first leveled off and then began to rise in 1989 due to numerous factors including drug abuse, AIDS epidemic, homelessness and a sharp fall in the number of preventative medicine clinics. The most alarming factor is the emergence of the multidrug resistance TB (MDR-TB). Resistance is indeed a serious problem, where prior to 1984 resistant cases were only 10% and this has since increased to 52%. TB could infect the lungs (80-85% of all cases) or other areas in the body (extrapulmonary TB) such as the bones and brain. Extrapulmonary TB is commo in HIV-positive patients and always develpos into an active infection.

Mycobacterium avium is another species that is ubiquitous, non pathogenic agent. However in immuno-compromised individuals it poses a serious opportunistic threat that may lead to life threatening conditions, known as Mycobacterium avium complex (MAC). It is now the third most common infection in these patients after infections with Candida sp. and Pneumocystis carinii. The difficulty in treating M. avium infections results from its presence inside macrophages.

Agents for TB:

One problem associated with Mycobacterium is the presence of a dormant stage that is more difficult to combat. In TB therapy, a combination of agents is usually employed, with various mechanisms of action aiming to lower incidence of resistance and possibly attack the dormant population of the mycobacterium.

Isoniazid (INH) , is a synthetic agent used since the 1950s.  It is bactericidal against replicating bacteria, and bacteriostatic against non-replicating bacteria.

Its mechanism of action is still debated, but it is obvious that it acts by inhibiting cell wall biosynthesis, since bacteria treated with this agent lose their acid fast ability.  It is converted to isonicotinic acid and isonicotinamide in bacterial cells via the action of a bacterial enzyme, katG .  It is proposed that the intermediates produced in this reaction are reactive acylating species, which are responsible for the antimicrobial activity. The acylating species are thought to interfere with Mycolic Acid biosynthesis, possibly through acylating NADH, a cofactor essential in the double bond reduction during the elongation of the fatty acid side chain.

This agent is orally absorbed, but food and antacids, especially those containing Aluminum interfere with absorption.  Metabolism takes place via an N-Acetyl transferase that yields inactive acetylated metabolites. Certain patients are fast acetylators of this drug and the dose will need to be adjusted.  Further metabolism will yield acetyl hydrazide, which has been implicated as a hepatotoxic compound.

The main mechanism for resistance development is the deletion of the gene responsible for the production of katG , preventing the production of the active species of INH.

Rifamycins are isolated from streptomyces sp. and are effective against a wide variety of bacteria, including mycobacterium, but cannot penetrate Gram negative cells.  They inhibit bacterial RNA polymerase by binding to the β-subunit of the enzyme, and block RNA synthesis. They are highly effective against rapidly dividing mycobacterium, and are specific to bacterial RNA polymerase.
Rifampin and Rifapentine are semisynthetic agents that are used in combination with INH. 

They bind reversibly to RNA polymerase via three binding points:

  1. Hydrophobic interaction between the naphthalene ring and proteins in the enzyme via a π-π interaction.
  2. Hydrogen bonding between hydroxyl groups on C21 and C23 with RNA polymerase.
  3. Hydrogen bonding between the hydroxyl groups at C1 and C8 and amino acids in the active site.
Both agents are orally active, but should be given on an empty stomach.  They have a red-orange color that will cause coloration of different fluids such as urine and tears and may stain the skin and contact lenses.
They induce some CYP450 isozymes, which may decrease the effectiveness of other drugs such as oral contraceptives, warfarin, other antimicrobials and many antiviral agents used in treatment of HIV.
Resistance arises via mutations to bacterial RNA polymerase, which diminishes the binding capacity of Rifamycins.
Rifapentine (Priftin®) is considered more active than Rifampin, and has a longer half-life resulting in a less frequent dose regimen. It
differs in the 3-substitution, which is thought to be responsible for cell entry, and thus this agent shows better oral bioavailability and has a superior dosing regimen.

Pyrazinamide is a popular drug used in many combinations, but unfortunately resistance develops very quickly.  Its mechanism of appears to involve its hydrolysis to pyrazinoic acid via the bacterial enzyme pncA.  The acid is believed to act as an antimetabolite of Nicotinamide and may interfere with NAD biosynthesis.  Its main action is through lowering the pH inside the organism to a deadly level.  It is especially effective against semi-dormant mycobacterium, and is used in combinations with INH and Rifampin.
Resistance arises by the absence of the enzyme, Pnc A.  The major side effect is a dose-related hepatotoxicity.

Ethambutol is effective against other mycobacterium such as M. avium an agent seen in many HIV patients. It inhibits arabinosyl transferase, an enzyme important in formation of the arabinogalactan portion of the mycobacterium cell wall. It shows synergism with Rifampin, possibly due to the cell wall damage it causes. Resistance arises as a result of over-expression of the enzyme.

Para-aminosalicylic acid acts by competing with PABA and inhibiting DHPS. It is metabolized by N-acetylation and may compete with INH for these acetylating enzymes. It is used with INH to lower the dose needed in fast acetylators. High incidence of resistance and its side effects have limited its use.

Other agents used in TB include Streptomycin, Kanamycin, Ethionamide (an analog of INH), Capreomycin (a cyclic peptide) and Cycloserine. Fluoroquinolones have also been shown to be effective especially Ciprofloxacin, Sparfloxacin and Ofloxacin and are often employed in multidrug resistant strains.

Agents Used for Leprosy:

Dapsone has the same mechanism of action and properties of Sulfonamides, inhibit DHPS and interfere with folic acid biosynthesis. It is however specific to Mycobacterium. It is well absorbed but is not very soluble and will cause GI irritation. A sulfoxane sodium is better tolerated but not easily metabolized to the active agent and thus three times the dose is needed.

Clofazimine is especially useful in MDR regimens. Its exact mechanism of action is unclear, but appears to involve generation of reactive oxidative and superooxide anions from neutrophil that may have antimicrobial effect. It is dark red in color and will stain the skin, urine, tears and faeces. The third agent that is widely used in Rifampin .

Macrolides are the first line agents for MAC infections . A combination of Clarithromycin or Azithromycin with Ethambutol is usually used for life in HIV pateints. Other agents used incude Rifampin, Ciprofloxacin and Amikacin.