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Transcript
PHL 424 Antimicrobials 4th Lecture By Abdelkader Ashour, Ph.D. Phone: 4677212 Email: [email protected] Cephalosporins, Overview Cephalosporins were first obtained from a filamentous fungus “Cephalosporium” Cephalosporins are similar to penicillins chemically, in mechanism of action and in toxicity The intrinsic antimicrobial activity of natural cephalosporins is low, but the attachment of various R1 and R2 groups (see classification) has yielded drugs of good therapeutic activity and low toxicity Cephalosporins are affected by the same resistance mechanisms as penicillins. However, they tend to be more resistant than the penicillins to bacterial b-lactamases, and therefore usually have a broader spectrum of activity Methicillin-resistant Staphylococcus aureus (MRSA) should be considered resistant to all cephalosporins Resistance to Cephalosporins I. Production of b-lactamases Bacteria can destroy b-lactam antibiotics enzymatically by a group of enzymes called b- lactamases The production of b-lactamases is considered the principal cause of bacterial resistance to b-lactam antibiotics The introduction of new classes of b-lactams has invariably been followed by the emergence of new b-lactamases capable of degrading them, as an example of rapid bacterial evolution under a rapidly changing environment Some b-lactamases are relatively substrate specific, and these are described as either penicillinases or cephalosporinases Other "extended spectrum" enzymes are less discriminant and can hydrolyze a variety of b-lactam antibiotics Resistance to Cephalosporins, I. Production of b-lactamases, contd. b-Lactamases are grouped into four classes: Class A b-lactamases include the extended-spectrum b-lactamases, which degrade penicillins, some cephalosporins and carbapenems Class B b-lactamases are Zn2+-dependent enzymes that destroy almost all b-lactams Class C b-lactamases are active against cephalosporins Class D includes cloxacillin-degrading enzymes. They also are active against some cephalosporins Class A and D enzymes are inhibited by the commercially available blactamase inhibitors, such as clavulanate and sulbactam Examples of bacteria that produce b-lactamases are staphylococcus aureus and many strains of H. influenzae, Neisseria and Pseudomonas Resistance to Cephalosporins II. The occurrence of PBPs with low affinity to cephalosporins The microorganism may be intrinsically resistant because of structural differences in the PBPs that are the targets of these drugs A sensitive strain may acquire resistance of this type by the development of high-molecular-weight PBPs that have decreased affinity for cephalosporins and penicillins, requiring clinically unattainable concentrations of the drug to effect its bactericidal activity Example 1: Methicillin-resistant S. aureus are resistant via acquisition of an additional high-molecular-weight PBP with a very low affinity for all b-lactam antibiotics Example 2: Cephalosporin resistance in Streptococcus pneumoniae is caused by altered PBPs (2 of the 5 high molecular weight PBPs) Resistance to Cephalosporins III. Decreased permeability to the drug Decreased penetration through the outer membrane prevents the drug from reaching the target PBP In G+ve bacteria, the peptidoglycan polymer is very near the cell surface, thus the small b-lactam antibiotic molecules can penetrate easily to the PBPs, where the final stages of the synthesis of the peptidoglycan take place G-ve organisms have an outer membrane that limits penetration of b-lactam antibiotics Some small hydrophilic antibiotics can diffuse through aqueous channels in the outer membrane that are formed by proteins called porins An extreme example is P. aeruginosa, which is intrinsically resistant to a wide variety of antibiotics because it lacks the classical high-permeability porins Active efflux pumps serve as another mechanism of resistance, removing the antibiotic from its site of action before it can act This is an important mechanism of b-lactam resistance in P. aeruginosa, E. coli and Neisseria gonorrhoeae Cephalosporins, Classification Cephalosporins are divided into four generations with original agents being referred to as first-generation cephalosporins, and the most recent agents as fourthgeneration cephalosporins In general, the spectrum of activity of cephalosporins increases with each generation because of decreasing susceptibility to bacterial b-lactamases First-Generation Cephalosporins Examples: cephradine, cephalexin (there are "longacting" agents, such as cefadroxil) They are active against most staphylococci, pneumococci, and all streptococci, with the important exception of enterococci Their activity against aerobic G-ve bacteria and against anaerobes is limited They act as penicillin G substitutes. They are resistant to b-lactamase They distribute widely throughout the body, but do not penetrate well into the CSF (not used for meningitis) Their t1/2 ranges from 30 minutes to 1.5 hours, and they are eliminated unchanged in the urine They should not be given to patients with a history of immediate-type hypersensitivity reactions to penicillins Cephalosporins, Classification Second-Generation Cephalosporins They have a broader bacteriologic spectrum than do the first-generation agents They are more resistant to b-lactamase than the first-generation drugs For example, cefamandole, cefuroxime, and cefaclor not only are more active against G-ve enteric bacteria but are active against both blactamase-negative and -positive strains of H. influenzae Their half-lives are similar to those of the firstgeneration agents Excretion is primarily renal, and they distribute widely. However, they do not attain sufficient concentrations in the CSF to warrant their use in the treatment of bacterial meningitis Cephalosporins, Classification Third-Generation Cephalosporins These agents retain much of the G+ve activity of the first two generations, although their antistaphylococcal activity is reduced. They are remarkably active against most G-ve enteric isolates Some third-generation cephalosporins (e.g., ceftazidime and cefoperazone) also are active against most isolates of P. aeruginosa In healthy subjects, their half-lives range from 1 hour (cefotaxime) to between 6 and 8 hours (ceftriaxone) These antibiotics diffuse well into most tissues (e.g., cefotaxime and ceftriaxone Excretion is primarily renal Indications include suspected bacterial meningitis and treatment of hospital-acquired multiple-resistant G-ve aerobic infections and suspected infections in certain compromised hosts Ceftriaxone is the drug of choice in treating infections caused by N. gonorrhoeae in geographic areas with a high incidence of penicillin-resistant isolates Cephalosporins, Classification Fourth-Generation Cephalosporins This generation of cephalosporins (such as cefepime, cefpirome) combines the antistaphylococcal activity (but only those that are methicillin-susceptible) of firstgeneration agents with the G-ve spectrum (including Pseudomonas) of thirdgeneration cephalosporins This class of cephalosporins have increased stability against hydrolysis by blactamases These drugs are usually administrated parenterally They demonstrated good penetration into CSF compared to cefotaxime They are highly active against common pediatric meningeal pathogens, including Streptococcus pneumoniae. Their vitro activity against penicillin-resistant pneumococci is generally twice that of cefotaxime or ceftriaxone Fourth-generation agents are particularly useful for the empirical treatment of serious infections in hospitalized patients when gram-positive microorganisms, Enterobacteriaceae, and Pseudomonas all are potential etiologies Cephalosporins, Classification Fifth-Generation Cephalosporins These are novel cephalosporins with activity against MRSA Example: Ceftaroline Ceftaroline, the active metabolite of a N-phosphono prodrug, ceftaroline fosamil, has been recently approved by the US FDA for the treatment of acute bacterial skin and skin structure infections and community-acquired bacterial pneumonia This antimicrobial agent binds to penicillin binding proteins (PBP) inhibiting cell wall synthesis and has a high affinity for PBP2a, which is associated with methicillin resistance Ceftaroline is consistently active against multidrug-resistant Streptococcus pneumoniae and Staphylococcus aureus, including methicillin-resistant strains The drug is usually administrated intravenously at 600 mg every 12 h Ceftaroline has low protein binding and is excreted by the kidneys and thus requires dose adjustments in individuals with renal failure Cephalosporins, Side Effects Serious, adverse reactions to the cephalosporins are uncommon. As with most antibiotics, the full spectrum of hypersensitivity reactions may occur, including rashes, fever, eosinophilia, serum sickness and anaphylaxis The incidence of immediate-type allergic reactions to the cephalosporins is increased among patients known to be allergic to penicillins Adverse reactions attributable to irritation at the site of administration are common. These reactions include local pain after i.m. injection, phlebitis after i.v. administration and minor GI complaints after oral administration Third-generation drugs may cause transient elevations of liver function test results and blood urea nitrogen concentrations. They also have a profound inhibitory effect on the vitamin K-synthesizing bacterial flora of the GIT. In addition, some agents, such as cefoperazone, can cause hypoprothombinemia and bleeding