Living Textbook MC610

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ß-Lactamase Inhibitors and Carbapenems

One of the problems associated with the use of Penicillins is the rising threat of resistance, which can occur by pumping out the antibiotic or a change in the structure of PBPs to lower the binding ability of Penicillins (such as the case with MRSA). But by far the most common cause of resistance is the production of ß-lactamases . In addition many of the more popular Penicillins are susceptible to ß-lactamases, so how can we combat this dangerous enemy?

  1. Develop Penicillins resistant to ß-lactamases, such as Methicillin and Dicloxacillin. Not enough agents are available
  2. Use a combination of ß-lactamases resistant penicillin and a ß-lactamases sensitive agent, which could theoretically be synergistic. Clinically proved to be disappointing, possibly due to poor cell wall penetration or induction of ß-lactamases.
  3. Inhibit the enzyme????

We have to understand that ß-lactamases are different enzymes depending on the organism. Generally they share the same mechanism of action of PBPs, but differ in their active site, required cation and origin. The mechanism of action involves a nucleophilic attack by an amino acid on the enzyme (in this case the ß-lactamase) on the ß-lactam ring, opening up the ring and followed by further destruction to the drug. Finally a molecule of water regenerates the enzyme and the degraded inactive Penicillin.

So one approach would be to develop mechanism-based inhibitors by creating a stable intermediate, which would tie up the enzyme.

Two types of inhibitors were designed, with the following general idea:

In Type I , a heterocyclic atom carries the resulting negative charge and resulting in a stable intermediate that is further stabilized by tautomerization.

In Type II , the presence of a 2,3-olefinic bond helps stabilizes the intermediate, through tautomerization. The carbapenems belong to this group and when first isolated and tested, they turned out to be effective as antimicrobial agents and thus are used alone.

Now if we employ the type I agents along side broad spectrum ß-lactamase sensitive agents, theoretically it should be a very efficient combination.

Type I Agents:

Clavulanic Acid , in (Augmentin® - Combination with Amoxicillin and Timentin® - Combination with Ticarcillin)

These combinations are used in a wide variety of infections ranging from skin, UTI, respiratory tract infections to ear infections. It showed a tremendous improvement over the use of the active antibiotic alone. For example, the MIC for Ampicillin in some resistant organisms is over 500 µg/ml, while in combination with Clavulanic acid, the MIC drops to 0.8 µg/ml.

Is Clavulanic acid, an acid stable compound? Yes, it does not have the amide side chain, thus can be used orally.

Note that in pharmaceutical preparations the dose of Clavulanic acid is constant, only the antibiotic dose changes.

Sulbactam is used in combination with Ampicillin (Unasyn®) in parenteral preparations - due to poor oral absorption - for a variety of infections. It cannot penetrate the Gram negative bacterial cell wall.

Tazobactam is a potent inhibitor used with Piperacillin (Zosyn®) mainly in pseudomonas infections. The combination is used parenterally (IV) in various infections including skin, abdominal and pelvic infections, as well as pneumonia of moderate severity.

Is the inhibition irreversible? Some studies seem to suggest that it is, where a second covalent bond is formed between the enzyme and the inhibitor.

Type II Agents:

Also known as Carbapenems. These are used alone and not in combination with other agents.

Thienamycin was originally isolated from streptomyces and tested as an antimicrobial agent, and it turned out to be effective against 98% of the 31,000 species it was tested against. It also was shown to inhibit most ß-lactamases.

It binds differently to PBPs, especially to PBP-2.

Carbapenems are eliminated through the renal tract, so adjustment for renal impairment patients is required. Resistance for these agents is low and evidence suggests that there exists little or no cross-resistance.

However, it is very unstable in both acidic and basic conditions, with the amine on one molecule attacking the ß-lactam ring on another molecule (intermolecular nucleophilic attack).

So modifications to this structure yielded Imipenem . The formimidoyl function is not basic enough to act as a nucleophile, and thus the compound is more stable. Both Imipenem and its parent compound penetrate the Gram negative cell wall efficiently through porins. So it was tried on UTI but showed disappointing clinical results. As it turns out it is hydrolyzed by renal dehydropeptidase (DHP-I) forming a nephrotoxic metabolite. So this drug was then combined with a DHP-I inhibitor, Cilistatin (Primaxin®).

Imipenem is not orally active and thus is used parenterally. It can penetrate most tissues and turned out to be a very efficient agent. It has the broadest spectrum of any antibiotic available in the United States, and is reserved for serious infections such as severe infections of the gastrointestinal tract infections, bones, skin...etc.

Carbapenems are eliminated through the renal tract, so adjustment for renal impairment patients is required. Resistance for these agents is low and evidence suggests that there exists little or no cross-resistance.

Meropenem (Merrem®) has recently been introduced. It is an effective agent and is not hydrolyzed by DHP-I possibly due to steric hinderness through the 1-Methyl group. It is claimed to have greater potency against Gram negative bacteria than Imipenem, and can be utilized in infections caused by multiresistant bacteria.

Ertapenem (Ivanz®) is a parenteral long acting Carbapenem, due to extensive protein binding. It is not effective against some pseudoomonal infections.

Doriepenem (Doribax®) is a new carbapenem with a broader spectrum of activity, in particular gainst pseudomonal infections.

Resistance against these agents is a serious problem as they are considered one of the last resort antibiotics we have. In particular, a Gram negative species, carbapenem-resistant Enterobacteriaceae (CRE), pose a serious threat due to acquiring potent beta‐lactamases such as the KPC (Klebsiella pneumoniae carbapenemase) and NDM (New Delhi Metallo‐beta‐lactamase).