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Doripenem is a carbapenem with in vitro antibacterial activity against aerobic and anaerobic Gram-positive and Gram-negative bacteria.

Mechanism of Action

Doripenem is an antibacterial drug.

Pharmacodynamics

Similar to other beta-lactam antimicrobial agents, the time that unbound plasma concentration of doripenem exceeds the MIC of the infecting organism has been shown to best correlate with efficacy in animal models of infection. However, the pharmacokinetic/pharmacodynamic relationship for doripenem has not been evaluated in patients.

In a randomized, positive- and placebo-controlled crossover QT study, 60 healthy subjects were administered DORIBAX® 500 mg IV every 8 hours × 4 doses and DORIBAX® 1g IV every 8 hours × 4 doses, placebo, and a single oral dose of positive control. At both the 500 mg and 1g DORIBAX® doses, no significant effect on QTc interval was detected at peak plasma concentration or at any other time.

Pharmacokinetics

Plasma Concentrations

Mean plasma concentrations of doripenem following a single 1-hour intravenous infusion of a 500 mg dose of DORIBAX® to 24 healthy subjects are shown below in Figure 1. The mean (SD) plasma Cmax and AUC0–∞ values were 23.0 (6.6) μg/mL and 36.3 (8.8) μg•hr/mL, respectively.

Figure 1: Average Doripenem Plasma Concentrations Versus Time Following a Single 1-Hour Intravenous Infusion of DORIBAX® 500 mg in Healthy Subjects (N=24)

The pharmacokinetics of doripenem (Cmax and AUC) are linear over a dose range of 500 mg to 1g when intravenously infused over 1 hour. There is no accumulation of doripenem following multiple intravenous infusions of either 500 mg or 1g administered every 8 hours for 7 to 10 days in subjects with normal renal function.

Distribution

The average binding of doripenem to plasma proteins is approximately 8.1% and is independent of plasma drug concentrations. The median (range) volume of distribution at steady state in healthy subjects is 16.8 L (8.09–55.5 L), similar to extracellular fluid volume (18.2 L).

Doripenem penetrates into several body fluids and tissues, including those at the site of infection for the approved indications. Doripenem concentrations in peritoneal and retroperitoneal fluid either match or exceed those required to inhibit most susceptible bacteria; however, the clinical relevance of this finding has not been established. Concentrations achieved in selected tissues and fluids following administration of DORIBAX^® are shown in Table 5:

Table 5: Doripenem Concentrations in Selected Tissues and Fluids

Metabolism

Metabolism of doripenem to a microbiologically inactive ring-opened metabolite (doripenem-M1) occurs primarily via dehydropeptidase-I. The mean (SD) plasma doripenem-M1-to-doripenem AUC ratio following single 500 mg and 1 g doses in healthy subjects is 18% (7.2%).

In pooled human liver microsomes, no in vitro metabolism of doripenem could be detected, indicating that doripenem is not a substrate for hepatic CYP450 enzymes.

Excretion

Doripenem is primarily eliminated unchanged by the kidneys. The mean plasma terminal elimination half-life of doripenem in healthy non-elderly adults is approximately 1 hour and mean (SD) plasma clearance is 15.9 (5.3) L/hour. Mean (SD) renal clearance is 10.8 (3.5) L/hour. The magnitude of this value, coupled with the significant decrease in the elimination of doripenem with concomitant probenecid administration, suggests that doripenem undergoes both glomerular filtration and active tubular secretion. In healthy adults given a single 500 mg dose of DORIBAX®, a mean of 70% and 15% of the dose was recovered in urine as unchanged drug and the ring-opened metabolite, respectively, within 48 hours. Following the administration of a single 500 mg dose of radiolabeled doripenem to healthy adults, less than 1% of the total radioactivity was recovered in feces after one week.

Special Populations

Patients with Renal Impairment

Following a single 500 mg dose of DORIBAX®, the mean AUC of doripenem in subjects with mild (CrCl 50–79 mL/min), moderate (CrCl 31–50 mL/min), and severe renal impairment (CrCl ≤ 30 mL/min) was 1.6-, 2.8-, and 5.1-times that of age-matched healthy subjects with normal renal function (CrCl ≥ 80 mL/min), respectively. Dosage adjustment is necessary in patients with moderate and severe renal impairment. [see DOSAGE AND ADMINISTRATION]

A single 500 mg dose of DORIBAX® was administered to subjects with end stage renal disease (ESRD) either one hour prior to or one hour after hemodialysis (HD). The mean doripenem AUC following the post-HD infusion was 7.8-times that of healthy subjects with normal renal function. The mean total recovery of doripenem and doripenem-M1 in the dialysate following a 4-hour HD session was 231 mg and 28 mg, respectively, or a total of 259 mg (52% of the dose). There is insufficient information to make dose adjustment recommendations in patients on hemodialysis.

Patients with Hepatic Impairment

The pharmacokinetics of doripenem in patients with hepatic impairment have not been established. As doripenem does not appear to undergo hepatic metabolism, the pharmacokinetics of doripenem are not expected to be affected by hepatic impairment.

Geriatric Patients

The impact of age on the pharmacokinetics of doripenem was evaluated in healthy male (n=6) and female (n=6) subjects ≥ 66 years of age. Mean doripenem AUC0-∞ was 49% higher in elderly adults relative to non-elderly adults. This difference in exposure was mainly attributed to age-related changes in creatinine clearance. No dosage adjustment is recommended for elderly patients with normal (for their age) renal function.

Gender

The effect of gender on the pharmacokinetics of doripenem was evaluated in healthy male (n=12) and female (n=12) subjects. Doripenem Cmax and AUC were similar between males and females. No dose adjustment is recommended based on gender.

Race

The effect of race on doripenem pharmacokinetics was examined using a population pharmacokinetic analysis of data from phase 1 and 2 studies. No significant difference in mean doripenem clearance was observed across race groups and therefore, no dosage adjustment is recommended based on race.

Drug Interactions

Administration of DORIBAX® 500 mg every 8 hours x 4 doses to 23 healthy male subjects receiving valproic acid 500 mg every 12 hours for 7 days decreased the mean Cmax of valproic acid by 44.5% (from 86.1 mcg/mL to 47.8 mcg/mL) and the mean Cmin by 77.7% (from 55.7 mcg/mL to 12.4 mcg/mL) compared to administration of valproic acid alone. The mean AUC0-tau of valproic acid also decreased by 63%. Conversely, the Cmax of the VPA-g metabolite was increased by 62.6% (from 5.19 mcg/mL to 8.44 mcg/mL) and the mean AUC0-tau of VPA-g was increased by 50%. The pharmacokinetics of doripenem were unaffected by the co-administration of valproic acid. [see WARNINGS AND PRECAUTIONS and DRUG INTERACTIONS]

Probenecid interferes with the active tubular secretion of doripenem, resulting in increased plasma concentrations. Probenecid increased doripenem AUC by 75% and prolonged the plasma elimination half-life by 53%. [see also DRUG INTERACTIONS]

In vitro studies in human liver microsomes and hepatocytes indicate that doripenem does not inhibit the major cytochrome P450 isoenzymes (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4/5, and CYP4A11). Therefore,

DORIBAX® is not expected to inhibit the clearance of drugs that are metabolized by these metabolic pathways in a clinically relevant manner.

DORIBAX® is also not expected to have CYP1A2, CYP2B6, CYP2C9, CYP2C19, CYP3A4/5, or UGT1A1 enzyme-inducing properties based on in vitro studies in cultured human hepatocytes.

Microbiology

Mechanism of Action

Doripenem belongs to the carbapenem class of antimicrobials. Doripenem exerts its bactericidal activity by inhibiting bacterial cell wall biosynthesis. Doripenem inactivates multiple essential penicillin-binding proteins (PBPs) resulting in inhibition of cell wall synthesis with subsequent cell death. In E. coli and P. aeruginosa, doripenem binds to PBP 2, which is involved in the maintenance of cell shape, as well as to PBPs 3 and 4.

Mechanism(s) of Resistance

Bacterial resistance mechanisms that affect doripenem include drug inactivation by carbapenem-hydrolyzing enzymes, mutant or acquired PBPs, decreased outer membrane permeability and active efflux. Doripenem is stable to hydrolysis by most beta-lactamases, including penicillinases and cephalosporinases produced by Gram-positive and Gram-negative bacteria, with the exception of carbapenem hydrolyzing beta-lactamases. Although cross-resistance may occur, some isolates resistant to other carbapenems may be susceptible to doripenem.

Interaction with Other Antimicrobials

In vitro synergy tests with doripenem show doripenem has little potential to antagonize or be antagonized by other antibiotics (e.g., levofloxacin, amikacin, trimethoprimsulfamethoxazole, daptomycin, linezolid, and vancomycin).

Doripenem has been shown to be active against most isolates of the following microorganisms, both in vitro and in clinical infections. [see INDICATIONS AND USAGE]

Facultative Gram-negative microorganisms

Acinetobacter baumannii
Escherichia coli
Klebsiella pneumoniae
Proteus mirabilis
Pseudomonas aeruginosa

Facultative Gram-positive microorganisms

Streptococcus constellatus
Streptococcus intermedius

Anaerobic microorganisms

Bacteroides caccae
Bacteroides fragilis
Bacteroides thetaiotaomicron
Bacteroides uniformis
Bacteroides vulgatus
Peptostreptococcus micros

At least 90 percent of the following microorganisms exhibit an in vitro minimal inhibitory concentration (MIC) less than or equal to the susceptible breakpoint for doripenem of organisms of the same type shown in Table 6. The safety and efficacy of doripenem in treating clinical infections due to these microorganisms has not been established in adequate and well-controlled clinical trials.

Facultative Gram-positive microorganisms

Staphylococcus aureus (methicillin-susceptible isolates only)
Streptococcus agalactiae
Streptococcus pyogenes

Facultative Gram-negative microorganisms

Citrobacter freundii
Enterobacter cloacae
Enterobacter aerogenes
Klebsiella oxytoca
Morganella morganii
Serratia marcescens

Susceptibility Test Methods

When available, the clinical microbiology laboratory should provide the results of in vitro susceptibility test results for antimicrobial drugs used in local hospitals and practice areas to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting the most effective antimicrobial.

Dilution Techniques

Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized procedure. Standardized procedures are based on a dilution method^(1,3) (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of doripenem powder. The MIC values should be interpreted according to the criteria provided in Table 6.

Diffusion Techniques

Quantitative methods that require measurement of zone diameters provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure^(2,3) requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 10 μg of doripenem to test the susceptibility of microorganisms to doripenem. Results should be interpreted according to the criteria in Table 6.

Anaerobic Techniques

For anaerobic bacteria, the susceptibility to doripenem as MICs should be determined by standardized test methods⁽⁴⁾. The MIC values obtained should be interpreted according to the criteria in Table 6.

Table 6: Susceptibility Test Result Interpretive Criteria for Doripenem

A report of Susceptible indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antimicrobial compound in the blood reaches the concentrations usually achievable.

Quality Control

Standardized susceptibility test procedures require the use of laboratory control microorganisms to monitor the performance of the supplies and reagents used in the assay, and the techniques of the individuals performing the test. Standard doripenem powder should provide the MIC values provided in Table 7. For the diffusion techniques using a 10 μg doripenem disk, the criteria noted in Table 7 should be achieved.

Table 7: Acceptable Quality Control Ranges for Susceptibility Testing

Clinical Studies

Complicated Intra-Abdominal Infections

A total of 946 adults with complicated intra-abdominal infections were randomized and received study medications in two identical multinational, multi-center, double-blind studies comparing DORIBAX® (500 mg administered over 1 hour every 8 hours) to meropenem (1 g administered over 3–5 minutes every 8 hours). Both regimens allowed the option to switch to oral amoxicillin/clavulanate (875 mg/125 mg administered twice daily) after a minimum of 3 days of intravenous therapy for a total of 5–14 days of intravenous and oral treatment. Patients with complicated appendicitis, or other complicated intra-abdominal infections, including bowel perforation, cholecystitis, intra-abdominal or solid organ abscess and generalized peritonitis were enrolled.

DORIBAX® was non-inferior to meropenem with regard to clinical cure rates in microbiologically evaluable (ME) patients, i.e., in patients with susceptible pathogens isolated at baseline and no major protocol deviations at test of cure (TOC) visit, 25–45 days after completing therapy. DORIBAX® was also non-inferior to meropenem in microbiological modified intent-to-treat (mMITT) patients, i.e., patients with baseline pathogens isolated regardless of susceptibility. Clinical cure rates at TOC are displayed by patient populations in Table 8. Microbiological cure rates at TOC by pathogen in ME patients are presented in Table 9.

Table 8: Clinical Cure Rates in Two Phase 3 Studies of Adults with Complicated Intra-Abdominal Infections

Table 9: Microbiological Cure Rates by Infecting Pathogen in Microbiologically Evaluable Adults with Complicated Intra-abdominal Infections

Complicated Urinary Tract Infections, Including Pyelonephritis

A total of 1171 adults with complicated urinary tract infections, including pyelonephritis (49 percent of microbiologically evaluable patients) were randomized and received study medications in two multi-center, multinational studies. Complicated pyelonephritis, i.e., pyelonephritis associated with predisposing anatomical or functional abnormality, comprised 17% of patients with pyelonephritis. One study was double-blind and compared DORIBAX® (500 mg administered over 1 hour every 8 hours) to IV levofloxacin (250 mg admininstered every 24 hours). The second study was a non-comparative study but of otherwise similar design. Both studies permitted the option of switching to oral levofloxacin (250 mg administered every 24 hours) after a minimum of 3 days of IV therapy for a total of 10 days of treatment. Patients with confirmed concurrent bacteremia were allowed to receive 500 mg of IV levofloxacin (either IV or oral as appropriate) for a total of 10 to 14 days of treatment.

DORIBAX® was non-inferior to levofloxacin with regard to the microbiological eradication rates in microbiologically evaluable (ME) patients, i.e., patients with baseline uropathogens isolated, no major protocol deviations and urine cultures at test of cure (TOC) visit 5–11 days after completing therapy. DORIBAX® was also non-inferior to levofloxacin in microbiological modified intent-to-treat (mMITT) patients, i.e., patients with pretreatment urine cultures. Overall microbiological eradication rates at TOC and the 95% CIs for the comparative study are displayed in Table 10. Microbiological eradication rates at TOC by pathogen in ME patients are presented in Table 11.

Table 10: Microbiological Eradication Rates from the Phase 3 Comparative Study of Adults with Complicated Urinary Tract Infections, Including Pyelonephritis

Table 11: Microbiological Eradication Rates By Infecting Pathogen in Microbiologically Evaluable Adults with Complicated Urinary Tract Infections, Including Pyelonephritis

REFERENCES

1. Clinical and Laboratory Standards Institute (CLSI). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically; Approved Standard – 7^th ed. CLSI Document M7-A7. CLSI, 940 West Valley Rd., Suite 1400, Wayne, PA 19087, 2006.

2. CLSI. Performance Standards for Antimicrobial Disk Susceptibility Tests; Approved Standard – 9^th ed. CLSI Document M2-A9. CLSI, Wayne, PA 19087, 2006.

3. CLSI. Performance Standards for Antimicrobial Susceptibility Testing; 17^th Informational Supplement. CLSI document M100-S17. CLSI, Wayne, PA 19087, 2007.

4. CLSI. Methods for Antimicrobial Susceptibility Testing of Anaerobic Bacteria; Approved Standard – 7^th ed. CLSI document M11-A7. CLSI, Wayne, PA 19087, 2007.

Last reviewed on RxList: 2/16/2012
This monograph has been modified to include the generic and brand name in many instances.