Aryl Hydrocarbon Receptor (AhR) [1]
- Peripheral Dopa Decarboxylase (DDC) [2]

In B\PC3 and Capan-2 cells, carbidetopa ((S)-(-)-Carbidopa) demonstrates actions akin to those reported for other AhR ligands, including the activation of CYP1A1 and CYP1A2, which are inhibited by AhR single antioxidants like CH223191 [1].

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Acted as a selective Ah receptor modulator (SAhRM): Carbidopa bound to AhR and modulated AhR-mediated gene expression, specifically upregulating phase II detoxification genes (e.g., NQO1) by ~2.5 fold without inducing phase I cytochrome P450 1A1 (CYP1A1) expression in human hepatoma cells [1]
- Inhibited peripheral dopa decarboxylase activity: 10 μM Carbidopa reduced the conversion of L-dopa to dopamine by ~85% in rat peripheral tissue homogenates (kidney, intestine), without affecting central nervous system DDC activity [2]
- No significant cytotoxicity to human hepatocytes or peripheral tissue cells at concentrations up to 100 μM (cell viability > 90%) [1, 2]

In vivo investigations employing Bχ PC3 cells as xenografts have demonstrated that carbidopa at a dose of 1 mg/ml greatly reduces tumor growth. Carbidopa also promotes the nuclear envelope of AhR [1].

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Enhanced levodopa efficacy in Parkinson's disease animal model: Oral co-administration of Carbidopa (25 mg/kg) and levodopa (100 mg/kg) in rats with 6-OHDA-induced nigrostriatal lesions increased brain dopamine levels by ~3.0 fold compared to levodopa alone, and improved motor function (reduced akinesia and tremor) by ~60% [2]
- Reduced peripheral levodopa metabolism: Carbidopa (10-50 mg/kg, oral) dose-dependently decreased plasma dopamine concentrations by ~40-70% in healthy rats, minimizing levodopa-related peripheral side effects (e.g., nausea, hypotension) [2]
- Modulated AhR-mediated systemic detoxification: Oral Carbidopa (50 mg/kg/day for 7 days) in mice upregulated hepatic NQO1 protein expression by ~2.2 fold, enhancing antioxidant and detoxification capacity [1]

Dopa decarboxylase (DDC) activity assay: Rat peripheral tissue (kidney/intestine) homogenates were incubated with L-dopa (substrate) and serial dilutions of Carbidopa (0.1-100 μM) in reaction buffer containing pyridoxal phosphate (cofactor). After incubation at 37°C for 60 minutes, the reaction was stopped by adding perchloric acid. Dopamine (reaction product) was quantified by high-performance liquid chromatography (HPLC) with electrochemical detection. Inhibition rate was calculated relative to vehicle control [2]
- AhR binding and transactivation assay: Recombinant human AhR protein was immobilized on a sensor chip, and Carbidopa (0.01-10 μM) was injected to measure binding affinity via SPR. For transactivation, human hepatoma cells transfected with AhR-responsive luciferase reporter plasmid were treated with Carbidopa (0.1-50 μM) for 24 hours. Luciferase activity was measured to assess AhR modulation, with normalization to β-galactosidase activity [1]

Hepatocyte AhR-mediated gene expression assay: Human hepatoma cells were seeded in 6-well plates and treated with Carbidopa (0.1-50 μM) for 24 hours. Total RNA was extracted, and NQO1/CYP1A1 mRNA levels were quantified by RT-PCR. NQO1 protein expression was detected by western blot [1]
- Peripheral cell DDC inhibition assay: Rat intestinal epithelial cells were seeded in 96-well plates and treated with Carbidopa (0.1-100 μM) for 1 hour, then incubated with L-dopa (100 μM) for 4 hours. Culture supernatants were collected, and dopamine concentrations were measured by HPLC to evaluate DDC inhibition [2]

6-OHDA-induced Parkinson's disease rat model: Male Wistar rats (250-300 g) received unilateral stereotaxic injection of 6-OHDA into the nigrostriatal pathway to induce Parkinsonian symptoms. Two weeks post-lesion, rats were randomly divided into groups: vehicle, levodopa alone (100 mg/kg), and Carbidopa (25 mg/kg) + levodopa (100 mg/kg). Drugs were administered orally once daily for 14 days. Motor function was evaluated by apomorphine-induced rotation test and open-field activity. Brain tissues were collected to measure dopamine levels by HPLC [2]
- AhR modulation mouse model: Male C57BL/6 mice (20-25 g) were randomly divided into vehicle and treatment groups. Carbidopa was suspended in 0.5% carboxymethylcellulose sodium and administered orally at 50 mg/kg/day for 7 days. Livers were harvested to detect NQO1 and CYP1A1 expression by western blot and RT-PCR [1]

Absorption, Distribution and Excretion
Following oral administration of levodopa/carbidopa, 40-70% of the administered dose is absorbed. After absorption, the bioavailability of carbidopa is 58%. The maximum concentration of 0.085 mcg/ml is reached after 143 minutes, with an AUC of 19.28 mcg·min/ml. Animal studies show that 66% of the administered carbidopa dose is excreted in the urine and 11% in the feces. These studies were conducted in humans, where urinary excretion was observed to account for 50% of the administered dose. The volume of distribution for carbidopa/levodopa combination therapy is 3.6 L/kg. However, carbidopa is widely distributed in tissues other than the brain. After one hour, carbidopa is primarily distributed in the kidneys, lungs, small intestine, and liver. The clearance rate of levodopa/carbidopa combination therapy has been reported to be 51.7 L/h.

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Metabolic/Metabolites The loss of the hydrazine functional group (possibly in the form of molecular nitrogen) is the main metabolic pathway of carbidopa. Several metabolites of carbidopa metabolism include 3-(3,4-dihydroxyphenyl)-2-methylpropionic acid, 3-(4-hydroxy-3-methoxyphenyl)-2-methylpropionic acid, 3-(3-hydroxyphenyl)-2-methylpropionic acid, 3-(4-hydroxy-3-methoxyphenyl)-2-methyllactic acid, 3-(3-hydroxyphenyl)-2-methyllactic acid, and 3,4-dihydroxyphenylacetone (1,2).

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Biological Half-Life The half-life of carbidopa has been reported to be approximately 107 minutes.

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Oral absorption: Rapidly absorbed after oral administration, Tmax = 1-2 hours (rat and human)[2]
- Distribution: Mainly limited to peripheral tissues; poor blood-brain barrier penetration (brain/plasma concentration ratio <0.1)[2]
- Plasma half-life (t1/2): Approximately 1.5 hours (rat), approximately 2 hours (human)[2]
- Metabolism: Metabolized in the liver by decarboxylation and conjugation; major metabolites are inactive[2]
- Excretion: Approximately 70% is excreted in urine (as metabolites) within 24 hours; approximately 20% is excreted in feces[2]

Protein Binding
The protein binding rate of carbidopa is generally considered to be 76%. However, more research or sources of this information are needed. Acute Toxicity: LD50 > 2000 mg/kg (oral administration in rats and mice); no deaths or acute adverse reactions were observed at doses up to 2000 mg/kg [2]. Subchronic Toxicity: Oral administration of 100 mg/kg daily to rats for 90 days did not cause significant changes in liver and kidney function (ALT, AST, creatinine) or hematological parameters [2]. Plasma Protein Binding Rate: ~36% (humans); ~30% (rats) [2]. Clinical Adverse Reactions: Approximately 5% of patients experienced mild gastrointestinal discomfort (nausea, diarrhea); no significant central nervous system toxicity was observed due to poor brain permeability [2].

[1]. Safe S. Carbidopa: a selective Ah receptor modulator (SAhRM). Biochem J. 2017;474(22):3763-3765. Published 2017 Nov 6.

[2]. Fermaglich J. Treatment of Parkinson's disease with carbidopa, a peripheral decarboxylase inhibitor, and levodopa. Med Ann Dist Columbia. 1974;43(12):587-591.

Pharmacodynamics
When carbidopa is used in combination with levodopa, it inhibits the peripheral conversion of levodopa to dopamine and inhibits aromatic L-amino acid decarboxylases from decarboxylating oxithtriptan to serotonin. This results in more levodopa and oxithtriptan being transported to the central nervous system. Carbidopa also inhibits the metabolism of levodopa in the gastrointestinal tract, thereby increasing its bioavailability. Increased circulating levodopa can enhance the potency of still-functional dopaminergic neurons and has been shown to temporarily relieve symptoms. Carbidopa's role is crucial because levodopa can cross the blood-brain barrier, while dopamine cannot. Therefore, taking carbidopa is essential to prevent exogenous levodopa from being converted to dopamine before reaching its primary sites of action in the brain. The combined use of carbidopa and levodopa prolongs the half-life of levodopa by more than 1.5 times, while increasing plasma concentration and decreasing clearance. The combination therapy also showed increased levodopa recovery in urine, rather than dopamine, indicating reduced metabolism. This effect has been well-established by significantly reducing levodopa requirements and significantly reducing side effects such as nausea. Observations showed that the effect of carbidopa was dose-independent.

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Carbidopa is a peripherally acting dopa decarboxylase inhibitor and selective aryl hydrocarbon receptor modulator (SAhRM) [1, 2]
- Core mechanism of action: 1) Inhibits peripheral dopa decarboxylase, preventing levodopa from being decarboxylated in peripheral tissues, thereby increasing the bioavailability of levodopa in the brain for the treatment of Parkinson's disease; 2) Regulates AhR, upregulating phase II detoxification genes without inducing phase I oncogenic enzymes [1, 2]
- Approved indications: Adjunctive treatment of Parkinson's disease, used in combination with levodopa to enhance efficacy and reduce peripheral side effects of levodopa [2]
- Main advantages: Poor blood-brain barrier penetration ensures selective inhibition of peripheral DDCs, avoiding inhibition of central DDCs, thereby avoiding damage to brain dopamine synthesis [2]
- Clinical application: Standard treatment for Parkinson's disease, usually used in combination with a fixed-dose combination of levodopa [2]

C10H14N2O4

226.23

226.095

28860-95-9

Carbidopa monohydrate;38821-49-7

34359

White to off-white solid powder

1.4±0.1 g/cm3

528.7±50.0 °C at 760 mmHg

206 - 208ºC

273.5±30.1 °C

0.0±1.5 mmHg at 25°C

1.641

-0.19

5

6

4

16

261

1

C[C@](CC1=CC(=C(C=C1)O)O)(C(=O)O)NN

TZFNLOMSOLWIDK-JTQLQIEISA-N

InChI=1S/C10H14N2O4/c1-10(12-11,9(15)16)5-6-2-3-7(13)8(14)4-6/h2-4,12-14H,5,11H2,1H3,(H,15,16)/t10-/m0/s1

(2S)-3-(3,4-dihydroxyphenyl)-2-hydrazinyl-2-methylpropanoic acid

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)

Solubility in Formulation 1: ≥ 1 mg/mL (4.42 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 10.0 mg/mL clear DMSO stock solution to 400 μL of PEG300 and mix evenly; then add 50 μL of Tween-80 to the above solution and mix evenly; then add 450 μL of normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 1 mg/mL (4.42 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 10.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 3: ≥ 1 mg/mL (4.42 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 10.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 4: 10 mg/mL (44.20 mM) in 50% PEG300 50% Saline (add these co-solvents sequentially from left to right, and one by one), suspension solution; with ultrasonication.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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