Enzymes dehydropeptidase

Imipenem-cilastatin is only available for intramuscular or intravenous administration because oral bioavailability is poor. The enzyme, dehydropeptidase 1, present in renal tubules, converts imipenem to an inactive metabolite. To decrease metabolic clearance, imipenem is combined with cilastatin, an inhibitor of dehydropeptidase I. Additional pharmacokinetic information appears in Table 45.2. 

Mechanism of Action A fixed-combination carbapenem. Imipenem penetrates the bacterial cell membrane and binds to penicillin-binding proteins, inhibiting cell wall synthesis. Cilastatin competitively inhibits the enzyme dehydropeptidase, preventing renal metabolism of imipenem. Therapeutic Effect Produces bacterial cell death. Pharmacokinetics Readily absorbed after IM administration. Protein binding 13%-21%. Widely distributed. Metabolized in the kidneys. Primarilyexcreted in urine. Removed by hemodialysis. Half-life 1 hr (increased in impaired renal function). 

Thienamycin and its derivatives are exciting new antibiotics. Then-clinical use is limited, however, by their susceptibility to the kidney enzyme dehydropeptidase I. Reversible inhibition of this enzyme is provided by cilastatin [11]. The preparation of the S-cyclopropane portion [10] of cilastatin is achieved (16) by decomposition of ethyl diazoacetate in isobutylene [9] in the presence of the chiral copper catalyst R-7644. The product [10] is obtained in 92% e.e. and then further processed to cilastatin. Cilastatin is now marketed in combination with the thienamycin derivative imipenem as a very-broad-spectnim antibiotic. 

Imipenem, a carbapenem antimicrobial, also possesses nephrotoxic potential. In animal models, nephrotoxicity is dose dependent and characterized by tubular necrosis. Interestingly, imipenem nephrotoxicity is markedly attenuated by co-administration of cilastatin, an inhibitor of the cytosolic and brush border enzyme dehydropeptidase I (DHP). Although DHP is responsible for hydrolyzing imipenem to inactive metabolites, the major protective effect of cilastatin appears to be due to inhibition of renal imipenem accumulation rather than DHP inhibition. 

In addition to variable chemical stabiUty the carbapenems are susceptible to P-lactam cleavage by a dehydropeptidase en2yme (DHP-I) located on the bmsh borders of the kidney (53). Clinically, MK 0787 (18) is used with an inhibitor of this en2yme, cil a sta tin [78852-98-9] (MK 0791) (34), 16 26 2 5 dramatic effect not only on the urinary recovery of the drug, but also reduces any nephrotoxic potential (52) (see Enzyme... 

In terms of activity there seems little to prevent some of these compounds finding a place in therapy, especially those such as SCH 29482, SUN 5555, and FCE 25199 which have oral absorption properties. However, as is the case for the carbapenems, some penems ate extensively metabolized by human renal dehydropeptidase-1 enzyme (144). Although no penem has received approval for clinical use as of this writing, expectations ate high that future research and development will change that. 

Thienamycin (Fig. 5.5E) is a broad-spectrum /3-lactam antibiotic with high /3-lactamase resistance. Unfortunately, it is chemically unstable, although the TV-formimidoyl derivative, imipenem, overcomes this defect. Imipenem (Fig. 5.5E) is stable to most/3-lactamases but it readily hydrolysed by kidney dehydropeptidase and is administered with a dehydropeptidase inhibitor, cilastatin. Meropenem, which has yet to be marketed, is more stable than imipenem to this enzyme and may thus be administered without cilastatin. Its chemical structure is depicted in Fig. 5.5F. 

This enzyme [EC 3.5.1.14] (also referred to as histozyme, hippuricase, benzamidase, dehydropeptidase II, amino-acylase I, and acylase I) catalyzes the hydrolysis of an A-acyl-L-amino acid to yield a fatty acid anion and an L-amino acid. The enzyme has a wide specificity for the amino acid derivative. It will also catalyze the hydrolysis of dehydropeptides. 

There are many dipeptidases [EC 3.4.13.x]. Cytosol nonspecific dipeptidase [EC 3.4.13.18] (also referred to as peptidase A, glycylglycine dipeptidase, glycylleucine dipeptidase, and A -)3-alanylarginine dipeptidase) catalyzes the hydrolysis of dipeptides. Membrane dipeptidase [EC 3.1.13.19] (also known as microsomal dipeptidase, renal dipeptidase, and dehydropeptidase I) is a zinc-dependent enzyme (a member of the peptidase family M19) that also catalyzes the hydrolysis of dipeptides. 

In humans, imipenem was foimd to be metabolized by an enzyme in the kidney, renal dehydropeptidase-I, which acts as a P-lactamase. Since the enzyme appears to serve no essential role in human metabolism, scientists were able to develop a synthetic competitive inhibitor, cilastatin, which they then used with imipenem to produce the combination drug, primaxin (Tienam). Primaxin was introduced into medical practice in 1985. 

Of the many derivatives prepared, BRL 42715 (31) was the compound with the best overall activity and stability. The enzyme, renal dehydropeptidase I (RDHP) is known to be a major cause of metabolic inactivation of carbapenems, such as imipenem, and penems. Whereas the ethylidene derivative (90c) showed substantial degradation of the biologically active (5R)-enantiomer in the presence of human kidney homogenate, BRL 42715 proved particularly stable, with 68% surviving after I hour s exposure to human kidney [90,91], In addition, BRL 42715 was only moderately bound (68%) to human serum. 

In the last 25 years, various natural carbapenems have been discovered (1). However, their potential is limited by chemical instability. Imipenem (N-formimidoylthienamycin), the first carbapenem in use, is therefore a stabilized synthetic compound. To overcome a second difficulty, namely inactivation by a kidney dehydropeptidase, imipenem has to be combined with cilastatin, a competitive inhibitor of that enzyme. Meropenem has better stability in the presence of renal dehydropeptidase I (2). The... 

Some cyclopropanes have proved to be useful as pharmaceutical intermediates. The compound, (-i-)-S-2,2-dimethylcyclopropanecarboxyHc acid, is a component of cilastatin (Fig. 3), which is administrated in combination with imi-penem, a carbapenem antibiotic [7]. In spite of its high and wide antibacterial activity, imipenem is found to be easily decomposed in the kidneys. This metabolism is suppressed by cilastatin, an enzyme inhibitor for dehydropeptidase I. 

Imipenem is rapidly inactivated by renal dehydropeptidase I and is administered in fixed combination with cilastatin, an inhibitor of this enzyme. Cilastatin increases the plasma half-life of imipenem and inhibits the formation of a potentially nephrotoxic metabolite. 

Imipenem, as well as thienamycin, penetrates very well through porins and is very stable, even inhibitory, to many (3-lactamases. Imipenem is not, however, orally active. When used to treat urinary tract infections, renal dehydropeptidase-1 hydrolyzes imipenem through hydrolysis of the (3-lactam and deactivates it. An inhibitor for this enzyme, cilastatin, is coadministered with imipenem to protect it. Inhibition of human dehydropeptidase does not seem to have deleterious consequences to the patient, making this combination highly efficacious against urinary tract infections. 

In 1984, Merck launched imipenem (N-formimidoylthienamycin). Imipenem acts as a nephrotoxin by interaction with the renal dehydropeptidase-1 (an enzyme of the kidney, which cleaves dipeptides and metabolises imipenem). For this reason, imipenem must be administered together with cilastatin [54, 55], which blocks dehydropeptidase-1. 

Dehydropeptidases. A widely distributed type of activity has been found to attack a group of substrates that are not known to occur naturally, the dehydropeptides. At least two enzymes with different specificities exist in animal tissues. The net reaction requires two equivalents O (JMt O uUs... 

According to Leuthardt and Greenstein (78), the system exocystine desulfhydrase-dehydropeptidase is the only intracellular enzyme system known to date which is present in most normal tissues but always alisent in tumors of the same tissues. 

Compound 6c, also known as Sanfetrinem combines a particularly broad spectrum (including gram-negative and gram-positive aerobes and anaerobes) with high potency, resistance to p-lactamases, the bacterial enzymes that hydrolyze p-lactam antibiotics, and stability to dehydropeptidases. 

In 1985, the self-inactivating property of thienamycin was overcome by the use of a terminal imino functionality which is less nudeophilic. Imipenem (Fig. 22.32) has a broad spectrum of activity and is administered by deep intramuscular injection or as an intravenous infusion. Renal dehydropeptidase 1, an enzyme present in the kidney, attacks and inactivates imipenem. This problem has been overcome by the co-administration of imipenem with cilastatin, a renal dehydropeptidase 1 inhibitor. Mer-openem (Fig. 22.32), an analogue introduced in 1996, is stable to the action of renal dehydropeptidase 1 and is also administered by deep intramuscular injection or as an intravenous infusion. Since the discovery of thienamycin in 1976, only parenteral analogues have heen successfully used clinically because the carhapenem structure is unstable in both the stomach and intestine. 

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