Considerations in dosage selection for third generation cephalosporins

Commonly used antibacterial and antifungal agents for hospitalised paediatric patients: implications for therapy with an emphasis on clinical pharmacokinetics

Due to normal growth and development, hospitalised paediatric patients with infection require unique consideration of immune function and drug disposition. Specifically, antibacterial and antifungal pharmacokinetics are influenced by volume of distribution, drug binding and elimination, which are a reflection of changing extracellular fluid volume, quantity and quality of plasma proteins, and renal and hepatic function. However, there is a paucity of data in paediatric patients addressing these issues and many empiric treatment practices are based on adult data. The penicillins and cephalosporins continue to be a mainstay of therapy because of their broad spectrum of activity, clinical efficacy and favourable tolerability profile. These antibacterials rapidly reach peak serum concentrations and readily diffuse into body tissues. Good penetration into the cerebrospinal fluid (CSF) has made the third-generation cephalosporins the agents of choice for the treatment of bacterial meningitis. These drugs are excreted primarily by the kidney. The carbapenems are broad-spectrum beta-lactam antibacterials which can potentially replace combination regimens. Vancomycin is a glycopeptide antibacterial with gram-positive activity useful for the treatment of resistant infections, or for those patients allergic to penicillins and cephalosporins. Volume of distribution is affected by age, gender, and bodyweight. It diffuses well across serous membranes and inflamed meninges. Vancomycin is excreted by the kidneys and is not removed by dialysis. The aminoglycosides continue to serve a useful role in the treatment of gram-negative, enterococcal and mycobacterial infections. Their volume of distribution approximates extracellular space. These drugs are also excreted renally and are removed by haemodialysis. Passage across the blood-brain barrier is poor, even in the face of meningeal inflammation. Low pH found in abscess conditions impairs function. Toxicity needs to be considered. Macrolide antibacterials are frequently used in the treatment of respiratory infections. Parenteral erythromycin can cause phlebitis, which limits its use. Parenteral azithromycin is better tolerated but paediatric pharmacokinetic data are lacking. Clindamycin is frequently used when anaerobic infections are suspected. Good oral absorption makes it a good choice for step-down therapy in intra-abdominal and skeletal infections. The use of quinolones in paediatrics has been restricted and most information available is in cystic fibrosis patients. High oral bioavailability is also important for step-down therapy. Amphotericin B has been the cornerstone of antifungal treatment in hospitalised patients. Its metabolism is poorly understood. The half-life increases with time and can be as long as 15 days after prolonged therapy. Oral absorption is poor. The azole antifungals are being used increasingly. Fluconazole is well tolerated, with high bioavailability and good penetration into the CSF. Itraconazole has greater activity against aspergillus, blastomycosis, histoplasmosis and sporotrichosis, although it's pharmacological and toxicity profiles are not as favourable.

Pharmacological properties of parenteral cephalosporins: rationale for ambulatory use

Parenteral cephalosporins are among the most frequently used antibiotics in hospital therapy. They are characterised by an extended spectrum of activity against gram-positive and gram-negative bacteria, and some also have good activity against anaerobes. They kill proliferating bacterial cells rapidly, and generally show only a low tendency to select resistant mutants. However, there are cephalosporin compounds which induce cephalosporinases very rapidly in certain microorganisms. Together with other beta-lactam antibiotics, parenteral cephalosporins interfere with bacterial cell wall synthesis by inhibiting peptidoglycan cross-linkage. Because of this specific target, they are nontoxic to mammalian cells, and have a very favourable adverse effect profile. The chemical stability of parenteral cephalosporins in aqueous solution is good. After intravenous injection, high concentrations of these agents are achieved in serum and tissue. Most cephalosporins are eliminated unchanged via the kidney, with a half-life of 1 to 2 hours. But there are also derivatives with a serum half-life of more than 2 and up to 8 hours, allowing 12- or 24-hour dosage intervals. Because of their reliable efficacy and low risk of adverse effects, the parenteral cephalosporins offer a high degree of tolerability even in the setting of outpatient antibiotic therapy. In particular, the derivatives of the third generation are characterised by unique pharmacological properties.

Antimicrobial prophylaxis in surgery: the role of pharmacokinetics

Even though surgical infection rates have decreased dramatically during the past 25 years, morbidity and mortality of infection in surgical treatment remains substantial. From a pharmacological point of view, the key factor of the efficacy of antibiotic prophylaxis is to attain bactericidal levels of antibiotic in serum and tissues (target site) during the whole intraoperative and early postoperative period. The success of antibiotic prophylaxis is assured only when the chosen antibiotic with a targeted spectrum and high antimicrobial efficacy is available at the critical moment, at the correct site and in sufficiently high concentration to prevent bacterial contamination of the surgical area. It would be desirable for reasons of convenience and cost if a single preoperative administration were sufficient. The pharmacokinetics and the half-life of antibiotics in the serum are directly related to the duration of activity of antibiotic in the tissue. Antibiotics with longer half-lives maintain levels in the tissues for longer periods than do antibiotics with shorter half-lives and they cover with a single dose the time required for prophylaxis even for longer operations. Finally, the application of the pharmacokinetic properties of antibiotics to surgical prophylaxis can provide the surgeon with certainty that adequate coverage and protection with antibiotics are achieved before and throughout the operation.

Pharmacokinetics of drugs used in critically ill adults

Critically ill patients exhibit a range of organ dysfunctions and often require treatment with a variety of drugs including sedatives, analgesics, neuromuscular blockers, antimicrobials, inotropes and gastric acid suppressants. Understanding how organ dysfunction can alter the pharmacokinetics of drugs is a vital aspect of therapy in this patient group. Many drugs will need to be given intravenously because of gastrointestinal failure. For those occasions on which the oral route is possible, bioavailability may be altered by hypomotility, changes in gastrointestinal pH and enteral feeding. Hepatic and renal dysfunction are the primary determinants of drug clearance, and hence of steady-state drug concentrations, and of efficacy and toxicity in the individual patient. Oxidative metabolism is the main clearance mechanism for many drugs and there is increasing recognition of the importance of decreased activity of the hepatic cytochrome P450 system in critically ill patients. Renal failure is equally important with both filtration and secretion clearance mechanisms being required for the removal of parent drugs and their active metabolites. Changes in the steady-state volume of distribution are often secondary to renal failure and may lower the effective drug concentrations in the body. Failure of the central nervous system, muscle, the endothelial system and endocrine system may also affect the pharmacokinetics of specific drugs. Time-dependency of alterations in pharmacokinetic parameters is well documented for some drugs. Understanding the underlying pathophysiology in the critically ill and applying pharmacokinetic principles in selection of drug and dose regimen is, therefore, crucial to optimising the pharmacodynamic response and outcome.

Ceftazidime pharmacokinetics in preterm infants: effects of renal function and gestational age

OBJECTIVE

The objectives of this study were (1) to determine the effects of gestational age on ceftazidime pharmacokinetics in the preterm infant, (2) to relate these effects to changes in glomerular filtration rate (GFR), and (3) to establish appropriate dosage recommendations for preterm infants on day 3 of life.

METHODS

Multiple-dose pharmacokinetics of ceftazidime (administered twice daily in a 25 or 50 mg/kg body weight intravenous dose) were evaluated in 136 preterm infants on day 3 of life. Blood samples were collected from an arterial catheter 0, 1/2, 1, 2, 4, 8, and 12 hours after the intravenous dose. An HPLC method was used to determine ceftazidime concentrations in serum. The GFR was studied simultaneously by means of the 24-hour continuous inulin infusion technique.

RESULTS

The total body clearance, volume of distribution, and elimination serum half-life of ceftazidime (mean +/- SD) were 55.7 +/- 34.4 ml/hr (37.3 +/- 11.9 ml/hr/kg), 496 +/- 228 ml (350 +/- 96 ml/kg), and 6.95 +/- 2.32 hours, respectively. The mean +/- SD peak and trough levels were 114.9 +/- 39.4 and 33.9 +/- 17.8 mg/L. All infants had a serum trough level above 5 mg/L. Clearance and volume of distribution of ceftazidime and GFR increased significantly with increasing gestational age, whereas serum trough levels and serum half-life of ceftazidime decreased significantly with increasing gestational age. Ceftazidime clearance increased significantly with increasing GFR. Prenatal exposure to indomethacin resulted in significantly lower GFR values and ceftazidime clearances.

CONCLUSIONS

Dosage recommendations for ceftazidime administration in preterm infants during the first week of life should be based on gestational age and GFR. Additional adjustments in dosage are indicated in preterm infants who are exposed prenatally to indomethacin.

Clinical pharmacokinetics of cefotetan

Cefotetan is a 7-alpha-methoxy beta-lactam. A long serum half-life and resistance to beta-lactamase hydrolysis have made cefotetan an attractive chemotherapeutic agent, and the results of clinical trials worldwide have demonstrated its efficacy in a wide variety of clinical situations. Cefotetan can be administered intravenously (bolus or infusion) or intramuscularly with lidocaine (lignocaine) 0.5%. Mean peak plasma concentrations are almost linearly related to dose. The volume of distribution is between 8 and 13L and is not different from other cephalosporins. No accumulation is seen after repeated doses and no metabolite has been detected in either plasma or urine. Total body clearance is 1.8 to 2.9 L/h. Renal clearance accounts for about 64 to 84% of a dose, and 75% of a dose is excreted in the urine within 24 hours. The plasma elimination half-life is between 3 and 4 hours after intravenous and intramuscular doses. Half-life is considerably prolonged in patients with renal impairment (up to 10 hours). Cefotetan concentrations are likely to be active against susceptible bacteria in most tissues and body fluids. Breast milk and cerebrospinal fluid concentrations are low. The recommended dosage is 1g every 12 hours, increasing to 2g in severe infections and 3g in life-threatening infections. In surgical prophylaxis, a single dose of 2g is given with the induction of anaesthesia; an additional dose of 2g may be administered 12 hours later. In children over 6 months, the recommended dosage is 30 mg/kg given 12-hourly. In patients with a creatinine clearance of 10 to 40 ml/min (0.6 to 2.4 L/h), the dose is halved or the dosage interval is doubled. When creatinine clearance is less than 10 ml/min (0.6 L/h), the dose is quartered or the dosage interval quadrupled.