Pharmacokinetics and Drug Metabolism
Absorption
Oral Bioavailability:
Maximal plasma cefdinir concentrations occur 2 to 4 hours postdose following capsule or suspension administration. Plasma cefdinir concentrations increase with dose, but the increases are less than dose-proportional from 300 mg (7 mg/kg) to 600 mg (14 mg/kg). Following administration of suspension to healthy adults, cefdinir bioavailability is 120% relative to capsules. Estimated bioavailability of cefdinir capsules is 21% following administration of a 300 mg capsule dose, and 16% following administration of a 600 mg capsule dose. Estimated absolute bioavailability of cefdinir suspension is 25%. Cefdinir oral suspension of 250 mg/5 mL strength was shown to be bioequivalent to the 125 mg/5 mL strength in healthy adults under fasting conditions.
Effect of Food:
The Cmax and AUC of cefdinir from the capsules are reduced by 16% and 10%, respectively, when given with a high-fat meal. In adults given the 250 mg/5 mL oral suspension with a high-fat meal, the Cmaxand AUC of cefdinir are reduced by 44% and 33%, respectively. The magnitude of these reductions is not likely to be clinically significant because the safety and efficacy studies of oral suspension in pediatric patients were conducted without regard to food intake. Therefore, cefdinir may be taken without regard to food.
Cefdinir Capsules: Cefdinir plasma concentrations and pharmacokinetic parameter values following administration of single 300 and 600 mg oral doses of cefdinir to adult subjects are presented in the following table:
Mean (±SD) Plasma Cefdinir Pharmacokinetic Parameter Values Following Administration of Capsules to Adult Subjects
Dose Cmax
(mcg/mL) tmax
(hr) AUC
(mcg•hr/mL)
300 mg 1.6
(0.55) 2.9
(0.89) 7.05
(2.17)
600 mg 2.87
(1.01) 3
(0.66) 11.1
(3.87)
Cefdinir Suspension: Cefdinir plasma concentrations and pharmacokinetic parameter values following administration of single 7 and 14 mg/kg oral doses of cefdinir to pediatric subjects (age 6 months to 12 years) are presented in the following table:
Mean (±SD) Plasma Cefdinir Pharmacokinetic Parameter Values Following Administration of Suspension to Pediatric Subjects
Dose Cmax
(mcg/mL) tmax
(hr) AUC
(mcg•hr/mL)
7 mg/kg 2.3
(0.65) 2.2
(0.6) 8.31
(2.5)
14 mg/kg 3.86
(0.62) 1.8
(0.4) 13.4
(2.64)
Multiple Dosing:
Cefdinir does not accumulate in plasma following once- or twice-daily administration to subjects with normal renal function.
Distribution:
The mean volume of distribution (Vdarea) of cefdinir in adult subjects is 0.35 L/kg (±0.29); in pediatric subjects (age 6 months to 12 years), cefdinir Vdarea is 0.67 L/kg (±0.38). Cefdinir is 60% to 70% bound to plasma proteins in both adult and pediatric subjects; binding is independent of concentration.
Skin Blister:
In adult subjects, median (range) maximal blister fluid cefdinir concentrations of 0.65 (0.33 to 1.1) and 1.1 (0.49 to 1.9) mcg/mL were observed 4 to 5 hours following administration of 300 and 600 mg doses, respectively. Mean (±SD) blister Cmax and AUC(0 to ∞) values were 48% (±13) and 91% (±18) of corresponding plasma values.
Tonsil Tissue:
In adult patients undergoing elective tonsillectomy, respective median tonsil tissue cefdinir concentrations 4 hours after administration of single 300 and 600 mg doses were 0.25 (0.22 to 0.46) and 0.36 (0.22 to 0.8) mcg/g. Mean tonsil tissue concentrations were 24% (±8) of corresponding plasma concentrations.
Sinus Tissue:
In adult patients undergoing elective maxillary and ethmoid sinus surgery, respective median sinus tissue cefdinir concentrations 4 hours after administration of single 300 and 600 mg doses were <0.12 (<0.12 to 0.46) and 0.21 (<0.12 to 2) mcg/g. Mean sinus tissue concentrations were 16% (±20) of corresponding plasma concentrations.
Lung Tissue:
In adult patients undergoing diagnostic bronchoscopy, respective median bronchial mucosa cefdinir concentrations 4 hours after administration of single 300 and 600 mg doses were 0.78 (<0.06 to 1.33) and 1.14 (<0.06 to 1.92) mcg/mL, and were 31% (±18) of corresponding plasma concentrations. Respective median epithelial lining fluid concentrations were 0.29 (<0.3 to 4.73) and 0.49 (<0.3 to 0.59) mcg/mL, and were 35% (±83) of corresponding plasma concentrations.
Middle Ear Fluid:
In 14 pediatric patients with acute bacterial otitis media, respective median middle ear fluid cefdinir concentrations 3 hours after administration of single 7 and 14 mg/kg doses were 0.21 (<0.09 to 0.94) and 0.72 (0.14 to 1.42) mcg/mL. Mean middle ear fluid concentrations were 15% (±15) of corresponding plasma concentrations.
CSF:
Data on cefdinir penetration into human cerebrospinal fluid are not available.
Metabolism and Excretion:
Cefdinir is not appreciably metabolized. Activity is primarily due to parent drug. Cefdinir is eliminated principally via renal excretion with a mean plasma elimination half-life (t½) of 1.7 (±0.6) hours. In healthy subjects with normal renal function, renal clearance is 2 (±1) mL/min/kg, and apparent oral clearance is 11.6 (±6) and 15.5 (±5.4) mL/min/kg following doses of 300 and 600 mg, respectively. Mean percent of dose recovered unchanged in the urine following 300 and 600 mg doses is 18.4% (±6.4) and 11.6% (±4.6), respectively. Cefdinir clearance is reduced in patients with renal dysfunction (see Special Populations: PATIENTS WITH RENAL INSUFFICIENCY).
Because renal excretion is the predominant pathway of elimination, dosage should be adjusted in patients with markedly compromised renal function or who are undergoing hemodialysis (see DOSAGE AND ADMINISTRATION).
Special Populations:
Patients with Renal Insufficiency:
Cefdinir pharmacokinetics were investigated in 21 adult subjects with varying degrees of renal function. Decreases in cefdinir elimination rate, apparent oral clearance (CL/F), and renal clearance were approximately proportional to the reduction in creatinine clearance (CLcr). As a result, plasma cefdinir concentrations were higher and persisted longer in subjects with renal impairment than in those without renal impairment. In subjects with CLcr between 30 and 60 mL/min, Cmax and t½increased by approximately 2 fold and AUC by approximately 3 fold. In subjects with CLcr <30 mL/min, Cmax increased by approximately 2 fold, t½ by approximately 5 fold, and AUC by approximately 6 fold. Dosage adjustment is recommended in patients with markedly compromised renal function (creatinine clearance <30 mL/min; see DOSAGE AND ADMINISTRATION).
Hemodialysis:
Cefdinir pharmacokinetics were studied in 8 adult subjects undergoing hemodialysis. Dialysis (4 hours duration) removed 63% of cefdinir from the body and reduced apparent elimination t½ from 16 (±3.5) to 3.2 (±1.2) hours. Dosage adjustment is recommended in this patient population (see DOSAGE AND ADMINISTRATION).
Hepatic Disease:
Because cefdinir is predominantly renally eliminated and not appreciably metabolized, studies in patients with hepatic impairment were not conducted. It is not expected that dosage adjustment will be required in this population.
Geriatric Patients:
The effect of age on cefdinir pharmacokinetics after a single 300 mg dose was evaluated in 32 subjects 19 to 91 years of age. Systemic exposure to cefdinir was substantially increased in older subjects (N=16), Cmax by 44% and AUC by 86%. This increase was due to a reduction in cefdinir clearance. The apparent volume of distribution was also reduced, thus no appreciable alterations in apparent elimination t½ were observed (elderly: 2.2 ± 0.6 hours vs young: 1.8 ± 0.4 hours). Since cefdinir clearance has been shown to be primarily related to changes in renal function rather than age, elderly patients do not require dosage adjustment unless they have markedly compromised renal function (creatinine clearance <30 mL/min, see PATIENTS WITH RENAL INSUFFICIENCY, above).
Gender and Race:
The results of a meta-analysis of clinical pharmacokinetics (N = 217) indicated no significant impact of either gender or race on cefdinir pharmacokinetics.
Microbiology
As with other cephalosporins, bactericidal activity of cefdinir results from inhibition of cell wall synthesis. Cefdinir is stable in the presence of some, but not all, β-lactamase enzymes. As a result, many organisms resistant to penicillins and some cephalosporins are susceptible to cefdinir.
Cefdinir has been shown to be active against most strains of the following microorganisms, both in vitro and in clinical infections as described in INDICATIONS AND USAGE.
Aerobic Gram-Positive Microorganisms
Staphylococcus aureus (including β-lactamase producing strains)
NOTE: Cefdinir is inactive against methicillin-resistant staphylococci.
Streptococcus pneumoniae (penicillin-susceptible strains only)
Streptococcus pyogenes
Aerobic Gram-Negative Microorganisms
Haemophilus influenzae (including β-lactamase producing strains)
Haemophilus parainfluenzae (including β-lactamase producing strains)
Moraxella catarrhalis (including β-lactamase producing strains)
The following in vitro data are available, but their clinical significance is unknown.
Cefdinir exhibits in vitro minimum inhibitory concentrations (MICs) of 1 mcg/mL or less against (≥90%) strains of the following microorganisms; however, the safety and effectiveness of cefdinir in treating clinical infections due to these microorganisms have not been established in adequate and well-controlled clinical trials.
Aerobic Gram-Positive Microorganisms
Staphylococcus epidermidis (methicillin-susceptible strains only)
Streptococcus agalactiae
Viridans group streptococci
NOTE: Cefdinir is inactive against Enterococcus and methicillin-resistant Staphylococcus species.
Aerobic Gram-Negative Microorganisms
Citrobacter diversus
Escherichia coli
Klebsiella pneumoniae
Proteus mirabilis
NOTE: Cefdinir is inactive against Pseudomonas and Enterobacter species.
Susceptibility Tests
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 method1 (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of cefdinir powder. The MIC values should be interpreted according to the following criteria:
For organisms other than Haemophilus spp. and Streptococcus spp:
MIC (mcg/mL) Interpretation
≤1 Susceptible (S)
2 Intermediate (I)
≥4 Resistant (R)
For Haemophilus spp2:
MIC (mcg/mL) Interpretation
≤ 1 Susceptible (S)
*The current absence of data on resistant strains precludes defining any results other than Susceptible. Strains yielding MIC results suggestive of a nonsusceptible category should be submitted to a reference laboratory for further testing.
For Streptococcus spp:
Streptococcus pneumoniae that are susceptible to penicillin (MIC ≤ 0.06 mcg/mL), or streptococci other than S. pneumoniae that are susceptible to penicillin (MIC ≤ 0.12 mcg/mL), can be considered susceptible to cefdinir. Testing of cefdinir against penicillin-intermediate or penicillin-resistant isolates is not recommended. Reliable interpretive criteria for cefdinir are not available.
A report of Susceptible indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches the concentration usually achievable. A report of Intermediate indicates that the result should be considered equivocal, and, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where high dosage of drug can be used. This category also provides a buffer zone which prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of Resistant indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable; other therapy should be selected.
Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of laboratory procedures. Standard cefdinir powder should provide the following MIC values:
Microorganism MIC Range (mcg/mL)
Escherichia coli ATCC 25922 0.12 to 0.5
Haemophilus influenzae ATCC 49766* 0.12 to 0.5
Staphylococcus aureus ATCC 29213 0.12 to 0.5
*This quality control range is applicable only to H. influenzae ATCC 49766 tested by a broth microdilution procedure using HTM.
1
National Committee for Clinical Laboratory Standards. Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically, 4th ed. Approved Standard, NCCLS Document M7-A4, Vol 17(2). NCCLS, Villanova, PA, Jan 1997.
2
These interpretive standards are applicable only to broth microdilution susceptibility tests withHaemophilus spp. using Haemophilus Test Medium (HTM). ( SEE REFERENCE 1)
Diffusion Techniques
Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure3requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 5 mcg cefdinir to test the susceptibility of microorganisms to cefdinir.
Reports from the laboratory providing results of the standard single-disk susceptibility test with a 5 mcg cefdinir disk should be interpreted according to the following criteria:
For organisms other than Haemophilus spp. and Streptococcus spp4:
Zone Diameter (mm) Interpretation
≥20 Susceptible (S)
17 to 19 Intermediate (I)
≤16 Resistant (R)
For Haemophilus spp5:
Zone Diameter (mm) Interpretation*
*
The current absence of data on resistant strains precludes defining any results other than “Susceptible.” Strains yielding MIC results suggestive of a “nonsusceptible” category should be submitted to a reference laboratory for further testing.
≥20 Susceptible (S)
For Streptococcus spp:
Isolates of Streptococcus pneumoniae should be tested against a 1 mcg oxacillin disk. Isolates with oxacillin zone sizes ≥20 mm are susceptible to penicillin and can be considered susceptible to cefdinir. Streptococci other than S. pneumoniae should be tested with a 10 unit penicillin disk. Isolates with penicillin zone sizes ≥28 mm are susceptible to penicillin and can be considered susceptible to cefdinir.
As with standardized dilution techniques, diffusion methods require the use of laboratory control microorganisms to control the technical aspects of laboratory procedures. For the diffusion technique, the 5 mcg cefdinir disk should provide the following zone diameters in these laboratory quality control strains:
Organism Zone Diameter (mm)
*
This quality control range is applicable only to testing of H. influenzae ATCC 49766 using HTM.
Escherichia coli ATCC 25922 24 to 28
Haemophilus influenzae ATCC 49766* 24 to 31
Staphylococcus aureus ATCC 25923 25 to 32
3
National Committee for Clinical Laboratory Standards. Performance Standards for Antimicrobial Disk Susceptibility Tests, 6th ed. Approved Standard, NCCLS Document M2-A6, Vol 17(1). NCCLS, Villanova, PA, Jan 1997.
4
Because certain strains of Citrobacter, Providencia, and Enterobacter spp. have been reported to give false susceptible results with the cefdinir disk, strains of these genera should not be tested and reported with this disk.
5
These zone diameter standards are applicable only to tests with Haemophilus spp. using HTM. (SEE REFERENCE 2)
