β-lactam
Penicillin-Binding Protein 2 (PBP 2) of Staphylococcus aureus (IC₅₀ ~ 0.1 µg/ml at 4°C; IC₅₀ > 10 µg/ml at 37°C in competition assay due to rapid deacylation) [1]
Penicillin-Binding Protein 3 (PBP 3) of Staphylococcus aureus (IC₅₀ for direct labeling ~ 0.025 µg/ml; > 90% saturation at 0.1 µg/ml in competition assay) [1]
Penicillin-Binding Protein 1 (PBP 1) of Staphylococcus aureus (90% saturation at 0.06 µg/ml in direct labeling assay) [1]

In competition assays using membrane preparations of Staphylococcus aureus strain 209P, Cefaclor specifically bound to PBP 3, achieving > 90% saturation at a concentration of 0.1 µg/ml after 15 minutes of incubation at 37°C. Under the same conditions, binding to PBP 2 was minimal (9% and 34% saturation at 1 and 10 µg/ml, respectively). [1]
Using direct radiolabeling assays with [³H]Cefaclor on membrane samples, Cefaclor demonstrated high-affinity binding to multiple PBPs. The concentrations required for 90% saturation were: 0.06 µg/ml for PBP 1, 0.3 µg/ml for PBP 2, and 0.025 µg/ml for PBP 3 after 15 minutes of incubation at 37°C. [1]
The study identified a rapid deacylation property of Cefaclor when bound to PBP 2. The half-life of the Cefaclor acyl-PBP 2 complex was approximately 10 minutes at 37°C, compared to > 120 minutes at 4°C. This rapid release of the drug from PBP 2 explained the apparent low-affinity binding observed in competition assays performed at 37°C. [1]
The minimum inhibitory concentration (MIC) of Cefaclor against S. aureus strain 209P, determined by the macrodilution method in Mueller-Hinton broth, was 1 µg/ml. [1]
In growth inhibition studies, Cefaclor at concentrations of 0.05, 0.125, and 0.25 µg/ml (which specifically saturate PBP 3 but not PBP 2 in competition assays) did not cause cell lysis alone. However, when combined with clavulanic acid (5 µg/ml, which saturates PBP 2), Cefaclor at 0.05 µg/ml significantly enhanced cell lysis. Cefaclor alone at 0.25 µg/ml produced cell lysis similar to the combination. [1]
Analysis of two PBP 2 peptides (acidic and basic) separated by nonequilibrium pH gradient electrophoresis (NEPHGE) showed that both peptides bound Cefaclor with similar high affinity at 4°C (IC₅₀ ~ 0.1 µg/ml for both). At 37°C, the acidic peptide showed lower apparent affinity (IC₅₀ of 20.5 µg/ml) compared to the basic peptide (IC₅₀ of 2 µg/ml), likely due to differential deacylation rates. [1]

The binding affinity of Cefaclor to Penicillin-Binding Proteins (PBPs) was assessed using two primary methods: competition assays and direct radiolabeling assays. [1]
For competition assays, membrane or whole-cell preparations of S. aureus were pre-incubated with varying concentrations of unlabeled Cefaclor for a specified time (e.g., 30 minutes at 37°C). Subsequently, the samples were radiolabeled with [³H]penicillin G (5 µg/ml) for a defined period (e.g., 15 minutes at 37°C or 4°C). The reaction was stopped by adding denaturation buffer for electrophoresis. The bound PBPs were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and visualized by fluorography. The reduction in radiolabeling intensity of specific PBPs, measured by scanning densitometry, relative to a control sample (no pre-incubation with unlabeled drug), was used to calculate the percent saturation and inhibitory concentrations (IC₅₀, IC₉₀). The effect of temperature on binding and deacylation was specifically investigated by performing the radiolabeling step at 4°C (to slow deacylation) and 37°C. [1]
For direct radiolabeling assays, membrane samples were incubated directly with varying concentrations of [³H]Cefaclor (e.g., 0.025 to 1 µg/ml) for 15 minutes at 37°C. The reaction was stopped, and PBPs were separated by SDS-PAGE and visualized by fluorography to determine direct binding saturation levels. [1]
The deacylation rate (half-life) of the Cefaclor-PBP 2 complex was determined by pre-incubating a membrane sample with unlabeled Cefaclor (1 µg/ml) for 30 minutes at 37°C, followed by radiolabeling with [³H]penicillin G at either 37°C or 4°C for varying times (1 to 120 minutes). The increase in radiolabeled PBP 2 over time at 37°C indicated release of bound Cefaclor and its replacement by [³H]penicillin G, allowing calculation of the deacylation half-life. [1]

Absorption, Distribution and Excretion
Absorbed well after oral administration, unaffected by food intake. Approximately 60% to 85% of the drug is excreted unchanged in the urine within 8 hours, with the majority excreted within the first 2 hours. Metabolism/Metabolites No significant biotransformation occurs in the liver (approximately 60% to 85% of the drug is excreted unchanged in the urine within 8 hours). Biological Half-Life 0.6–0.9 hours

Effects During Pregnancy and Lactation
◉ Overview of Use During Lactation
Cefaclor has been discontinued in the United States. Limited information suggests that low concentrations of the drug in breast milk are not expected to have adverse effects on breastfed infants after mothers have taken cefaclor. There have been reports that cephalosporins occasionally disrupt the gut microbiota of infants, leading to diarrhea or thrush, but these effects have not been fully assessed. It is safe for breastfeeding women to take cefaclor.
◉ Effects on Breastfed Infants
In a telephone follow-up study, 5 breastfeeding mothers reported taking cefaclor (dosage not specified). One mother reported that her infant developed diarrhea. No rashes or candidiasis were reported in the exposed infants.
◉ Effects on Lactation and Breast Milk
As of the revision date, no relevant published information was found.

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Protein Binding 23.5%

[1]. Characterization of penicillin-binding protein 2 of Staphylococcus aureus: deacylation reaction and identification of two penicillin-binding peptides. Antimicrob Agents Chemother. 1992 Mar;36(3):656-61.

[2]. Cefaclor revisited. Clin Ther. 2000 Feb;22(2):154-66.

[3]. Pharmacokinetic profile of cefaclor. Int J Clin Pharmacol Ther .

Cefaclor is a cephalosporin with a chlorine atom at position 3 and an (R)-2-amino-2-phenylacetamide group at positions 7 of its cephalosporin backbone. It is both an antibacterial drug and a drug allergen. It is a semi-synthetic broad-spectrum antibiotic derivative of cephalexin. Anhydrous cefaclor is a cephalosporin antibacterial drug. Cefaclor is a second-generation β-lactam cephalosporin antibiotic with bactericidal activity. Cefaclor binds to and inactivates penicillin-binding protein (PBP) located on the bacterial cell membrane. PBP is an enzyme involved in the final stages of bacterial cell wall assembly and in remodeling the cell wall during bacterial growth and division. Inactivation of PBP interferes with the cross-linking of peptidoglycan chains, which is crucial for maintaining the strength and rigidity of the bacterial cell wall. This leads to weakening of the bacterial cell wall and cell lysis. Anhydrous cefaclor is the anhydrous form of cefaclor, a second-generation β-lactam cephalosporin with antibacterial activity. Cefaclor binds to and inactivates penicillin-binding proteins (PBPs) located on the inner membrane of bacterial cell walls. Inactivation of PBPs interferes with the cross-linking of peptidoglycan chains, which is crucial for maintaining the strength and rigidity of bacterial cell walls. This leads to weakened bacterial cell walls and cell lysis. Cefaclor is a semi-synthetic broad-spectrum antibiotic and a derivative of cefalexin. Indications Used to treat certain bacterial infections, such as pneumonia, as well as infections of the ear, lungs, skin, throat, and urinary tract. FDA Label Mechanism of Action Like penicillin antibiotics, cefaclor is a β-lactam antibiotic. It inhibits the third (and final) stage of bacterial cell wall synthesis by binding to specific penicillin-binding proteins (PBPs) located within the bacterial cell wall. Subsequently, bacterial cell wall autolysins (such as autolysins) mediate cell lysis. Cefaclor may interfere with the action of autolysin inhibitors.

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Pharmacodynamics
Cefaclor is a second-generation cephalosporin antibiotic with an antibacterial spectrum similar to first-generation cephalosporins. In vitro Studies have shown that the bactericidal effect of cephalosporins stems from the inhibition of cell wall synthesis. In vitro and in vivo clinical studies have demonstrated that cefaclor is effective against most Gram-positive aerobic bacteria—Staphylococci (including coagulase-positive, coagulase-negative, and penicillinase-producing strains), Streptococcus pneumoniae, Streptococcus pyogenes (Group A hemolytic streptococci) and Gram-negative aerobic bacteria—Escherichia coli, Haemophilus influenzae (including β-lactamase-producing ampicillin-resistant strains), Klebsiella spp., and Proteus mirabilis. Cefaclor is a β-lactam antibiotic. This study focuses on the interaction between cefaclor and penicillin-binding proteins (PBPs) in Staphylococcus aureus. [1]
A key finding is that cefaclor binds to PBP2 with high affinity, but this binding is characterized by rapid deacylation (drug release from the enzyme complex) at physiological temperatures (37°C) with a half-life of approximately 10 minutes. This characteristic could lead to an underestimation of its binding affinity in standard competitive assays if the radiolabeling step is prolonged. [1]
The study also found that PBP2 consists of two distinct penicillin-binding peptides (an acidic peptide and a basic peptide), which can be separated by isoelectric focusing, and both peptides can bind to cefaclor. [1]
The antibacterial activity of cefaclor against Staphylococcus aureus is related to its binding to PBP2 (as well as PBP1 and PBP3), resolving the previous paradox of cefaclor being effective even when it does not bind to PBP2 in conventional assays. [1]

C15H14CLN3O4S

367.81

367.039

C, 48.98; H, 3.84; Cl, 9.64; N, 11.42; O, 17.40; S, 8.72

53994-73-3

Cefaclor monohydrate;70356-03-5;Cefaclor-d5;1426173-90-1

51039

Light yellow to yellow solid powder

1.6±0.1 g/cm3

713.4±60.0 °C at 760 mmHg

385.2±32.9 °C

0.0±2.4 mmHg at 25°C

1.722

0.1

3

6

4

24

606

3

O=C(C(N12)=C(Cl)CS[C@]2([H])[C@H](NC([C@H](N)C3=CC=CC=C3)=O)C1=O)O

QYIYFLOTGYLRGG-GPCCPHFNSA-N

InChI=1S/C15H14ClN3O4S/c16-8-6-24-14-10(13(21)19(14)11(8)15(22)23)18-12(20)9(17)7-4-2-1-3-5-7/h1-5,9-10,14H,6,17H2,(H,18,20)(H,22,23)/t9-,10-,14-/m1/s1

(6R,7R)-7-[[(2R)-2-amino-2-phenylacetyl]amino]-3-chloro-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid

S6472; Cefaclor Anhydrous; Ceclor; Lilly 99638; Monohydrate, Cefaclor;Cefaclorum; Cephaclor; Raniclor; Kefral; Cefaclor Monohydrate; Keclor; S 6472; S-6472;

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO : ~18.5 mg/mL (~50.30 mM)
H2O : ~3.85 mg/mL (~10.47 mM)

Solubility in Formulation 1: 3.33 mg/mL (9.05 mM) in PBS (add these co-solvents sequentially from left to right, and one by one), clear solution; with sonication (<60°C).

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 (Please use freshly prepared in vivo formulations for optimal results.)