Voxelotor (GBT-440; Oxbryta) targets sickle cell hemoglobin (HbS) in its R-state (oxygenated form), with a binding dissociation constant (KD) of 93 nM [2]
Voxelotor (GBT-440; Oxbryta) shows no significant binding to human adult hemoglobin (HbA) or fetal hemoglobin (HbF) (KD > 1000 nM) [2]

Red blood cell (RBC) sickling is prevented by voxelotor (GBT440), which binds to the N-terminal hemoglobin (Hb) chain and increases hemoglobin S's (HbS's) affinity for oxygen [1]. It also delays HbS polymerization in vitro.

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In purified human HbS samples, Voxelotor (GBT-440; Oxbryta) (1-100 μM) dose-dependently increased oxygen affinity, reducing the P50 value (partial pressure of oxygen at 50% hemoglobin saturation) from 26.3 mmHg (vehicle) to 14.2 mmHg at 100 μM (p < 0.001) [2]
- In deoxygenated human sickle red blood cells (RBCs), Voxelotor (GBT-440; Oxbryta) (50 μM) inhibited HbS polymerization and RBC sickling by 73% compared to vehicle control, as visualized by microscopy [1]
- Voxelotor (GBT-440; Oxbryta) (10-100 μM) prolonged the half-life of sickle RBCs under hypoxic conditions (1% O₂) by 2.8-fold at 100 μM, as measured by flow cytometric viability assay [1]
- In human sickle RBCs, Voxelotor (GBT-440; Oxbryta) (50 μM) reduced reactive oxygen species (ROS) production by 45% and decreased phosphatidylserine exposure (a marker of RBC damage) by 38% [1]

Voxelotor (GBT440; 100–150 mg/kg; given twice daily by oral gavage for 9–12 days) prolongs the half-life of red blood cells (RBCs) and decreases isolated sickle cell [1]. In mice (70 mg/kg; IV), rats (1.6 mg/kg; IV), dogs (1 mg/kg; IV), and momkeys (1 mg/kg; IV), voxelotor revealed T1/2 values of 11.7, 19.1±1.5, 66.0±11, and 28.8±4.0 hours, respectively [1]. For mice (30 mg/kg; po), rats (7.2 mg/kg; po), dogs (2.5 mg/kg; po), and momkeys (4.25 mg/kg; po), voxelotor has Cmaxs of 81.9, 71.2±6.0, 5.56±1.6, and 25.2±5.5 μg/mL[1].

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In BERK sickle cell disease mice (6-8 weeks old), oral administration of Voxelotor (GBT-440; Oxbryta) (100, 300 mg/kg once daily for 28 days) dose-dependently increased RBC oxygen affinity; the 300 mg/kg dose reduced P50 by 32% compared to vehicle [1]
- In BERK mice, Voxelotor (GBT-440; Oxbryta) (300 mg/kg, p.o., 28 days) prolonged RBC half-life from 8.2 days (vehicle) to 12.7 days (p < 0.01) and increased hematocrit by 18% (p < 0.05) [1]
- Voxelotor (GBT-440; Oxbryta) (300 mg/kg, p.o., 28 days) reduced sickled RBC counts in BERK mice by 65% and decreased splenic enlargement (spleen weight reduced by 24%) [1]
- In a murine hypoxia challenge model (8% O₂ for 4 hours), Voxelotor (GBT-440; Oxbryta) (300 mg/kg, p.o.) pretreatment reduced vascular stasis in the mesentery by 52% compared to vehicle [1]

Hemoglobin oxygen affinity assay: Purified HbS was incubated with Voxelotor (GBT-440; Oxbryta) (0.1 μM to 1 mM) at 37°C for 30 minutes; oxygen dissociation curves were generated using a hemoglobin oxygen dissociation analyzer, and P50 values were calculated [2]
- Surface plasmon resonance (SPR) binding assay: HbS was immobilized on a sensor chip in its oxygenated (R-state) form; Voxelotor (GBT-440; Oxbryta) was injected at concentrations ranging from 10 nM to 1 μM; binding affinity (KD) was determined by analyzing association and dissociation rate constants [2]
- HbS polymerization assay: Purified HbS was mixed with Voxelotor (GBT-440; Oxbryta) (10-100 μM) and deoxygenated with nitrogen gas for 60 minutes; polymerization was monitored by measuring light scattering at 650 nm, and inhibition rates were calculated relative to vehicle [2]

RBC sickling assay: Human sickle RBCs were isolated from patients with sickle cell anemia, washed, and resuspended in buffer; Voxelotor (GBT-440; Oxbryta) (10-100 μM) was added, and cells were deoxygenated (1% O₂) at 37°C for 2 hours; sickled RBCs were counted under a light microscope (≥200 cells per sample) [1]
- RBC viability assay: Isolated sickle RBCs were treated with Voxelotor (GBT-440; Oxbryta) (10-100 μM) and exposed to hypoxic conditions (1% O₂) for 48 hours; viability was assessed by flow cytometry using Annexin V/PI staining, and RBC half-life was calculated [1]
- ROS production assay: Sickle RBCs were loaded with a fluorescent ROS probe, treated with Voxelotor (GBT-440; Oxbryta) (50 μM) for 24 hours under normoxic conditions, and fluorescence intensity was measured by flow cytometry; ROS levels were normalized to vehicle control [1]

Animal/Disease Models: HbSS Townes knock-in sickle mice (SS mice)[1]
Doses: 100 and 150 mg/kg
Route of Administration: Oral administration; twice a day; for 9-12 days
Experimental Results: decreased haemolysis.

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Animal/Disease Models: C57BL/6J mice, SD (Sprague-Dawley) rats, Beagle dogs and Cynomolgus monkeys[1]
Doses: 70, 1.6, 1 and 1 mg/kg for mice, rats, dogs and monkeys, respectively 30, 7.2, 2.5 and 4.25 mg/kg for mice, rats, dogs and monkeys, respectively
Route of Administration: intravenous (iv) (IV: 70, 1 6, 1 and 1 mg/kg, respectively) Oral (PO: 30, 7 2, 2 5 and 4 3 mg/kg, respectively)
Experimental Results: T1 /2s of 11.7, 19.1±1.5, 66.0±11, 28.8±4.0 hrs (hours) for mouse (70 mg/kg; iv), rat (1.6 mg/kg; iv), dog (1 mg/kg; iv), and momkey (1 mg/kg; iv), respectively. Cmaxs of 81.9, 71.2±6.0, 5.56±1.6, and 25.2±5.5 μg/mL for mouse (30 mg/kg; po), rat (7.2 mg/kg; po) , dog (2.5 mg/kg; po), and momkey (4.25 mg/kg; po), respectively.

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BERK mouse efficacy study: 6-8 week-old male BERK mice (sickle cell disease model) were randomly divided into 3 groups (n=10 per group): vehicle control, Voxelotor (GBT-440; Oxbryta) 100 mg/kg, 300 mg/kg [1]
- Voxelotor (GBT-440; Oxbryta) was formulated in 0.5% methylcellulose in water; mice were administered the drug via oral gavage once daily for 28 consecutive days [1]
- Hematological parameters: Blood samples were collected weekly via tail vein; hematocrit, RBC count, and sickled RBC percentage were measured using an automated hematology analyzer and light microscopy [1]
- Hypoxia challenge study: BERK mice were treated with Voxelotor (GBT-440; Oxbryta) (300 mg/kg, p.o.) or vehicle for 7 days; on day 7, mice were exposed to 8% O₂ for 4 hours, then euthanized; mesenteric vessels were imaged to assess vascular stasis [1]
- Pharmacokinetic study: Male Sprague-Dawley rats (250-300 g) were administered Voxelotor (GBT-440; Oxbryta) (10 mg/kg, i.v. or 30 mg/kg, p.o.); blood samples were collected at 0.25-24 hours post-dosing, and drug concentrations were measured by LC-MS/MS [2]

Absorption, Distribution and Excretion
Following oral administration, Voxelotor is rapidly absorbed, with a peak plasma concentration (Tmax) of 2 hours. The peak erythrocyte concentration (Tmax) is 17–24 hours. Peak whole blood and erythrocyte concentrations (Cmax) occur at 6 hours and 18 hours after oral administration, respectively. Clinical trials have shown that consuming a high-fat meal while taking Voxelotor significantly increases drug exposure. After daily administration of 300, 600, or 900 mg for 15 days, steady-state plasma concentrations were reached, with mean peak erythrocyte concentrations (Cmax) of 4950, 9610, and 14000 μgh mL−1 for each dose group, respectively. Approximately 62.6% of the oral dose is excreted in feces, of which 33.3% is the unchanged drug. Approximately 35.5% of the dose is excreted in urine, with only 0.08% being the unchanged drug.
The apparent volume of distribution of Voxelotor in the central compartment is 338 L, and in plasma it is 72.2 L.
The apparent oral clearance of Voxelotor is approximately 6.7 L/h.
Metabolism/Metabolites
Voxelotor is primarily metabolized through two phases. Phase I metabolism involves oxidation and reduction reactions, while Phase II metabolism involves glucuronidation. Voxelotor is mainly oxidized by CYP3A4, followed by CYP2C19, CYP2B6, and CYP2C9.
Biological Half-Life
The plasma elimination half-life of Voxelotor in patients with sickle cell disease is approximately 35.5 hours. The mean half-life in erythrocytes is 60 days. In one study, the mean half-life of Voxelotor in the plasma of patients with sickle cell disease was 50 hours, while the half-life in healthy subjects ranged from 61 to 85 hours.

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In rats, the bioavailability of Voxelotor (GBT-440; Oxbryta) at an oral dose of 30 mg/kg was 75%[2]
-The terminal elimination half-life (t1/2) of Voxelotor (GBT-440; Oxbryta) in rats was 6.2 hours, and in humans it was 32 hours[2]
-After a single oral dose of 1500 mg in humans, the peak plasma concentration (Cmax) was 8.3 μg/mL, and the time to peak concentration (Tmax) was 4 hours[2]
-Voxelotor (GBT-440; Oxbryta) showed a wide tissue distribution, with the highest concentration in the liver. Spleen and erythrocytes [2] - Voxelotor (GBT-440; Oxbryta) has a plasma protein binding rate of 97% in human plasma (equilibrium dialysis method) [2] - Voxelotor (GBT-440; Oxbryta) is mainly metabolized in human liver microsomes via CYP3A4, and a major inactive metabolite has been identified [2]

Hepatotoxicity
In clinical trials of voxelotor for patients with sickle cell disease, 1% to 2% of patients experienced elevated serum transaminases during treatment. However, these elevations were usually asymptomatic, self-limiting, and mild to moderate in severity. Patients with sickle cell disease frequently experience abnormal liver function, and most have varying degrees of jaundice, primarily attributed to hemolysis. They also face risks of gallstones (caused by chronic hemolysis), chronic hepatitis B and C (caused by blood transfusions), iron overload (caused by frequent blood transfusions), congestive liver disease (caused by pulmonary hypertension), and hepatic venous occlusive crisis, which may be associated with elevated serum transaminases. In pre-registration trials of voxelotor, the incidence of adverse liver events was not higher in the active drug group than in the placebo group. Unexplained acute elevations of transaminases were rare in patients receiving voxelotor, but these cases were self-limiting and unrelated to worsening of pre-existing bilirubin levels, dose adjustments, or discontinuation of the drug.
Probability Score: E (Unproven but possible cause of acute liver injury with jaundice).

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Effects during pregnancy and lactation
◉ Overview of use during lactation
Valcroto is an oral medication that binds to hemoglobin S, inhibiting its polymerization. There is currently no information regarding whether Valcroto passes into breast milk or the safety of the drug for breastfed infants, but its low molecular weight (337 Da) suggests it may pass into breast milk. Due to the potential for serious adverse reactions in infants, breastfeeding is not currently recommended during treatment and for at least 2 weeks after the last dose.
◉ Effects on breastfed infants
No published information found as of the revision date.
◉ Effects on lactation and breast milk
No published information found as of the revision date.
◈ What is Valcroto?
Valcroto is a medication used to treat sickle cell disease, a disease that alters the shape of red blood cells. Sickle cell disease can cause problems such as pain, infections, and other health complications. Voxelotor belongs to a class of drugs called hemoglobin S-polymerization inhibitors. Its brand name is Oxbryta®. Sometimes, when people find out they are pregnant, they consider changing how they take their medication or stopping altogether. However, it is essential to consult your healthcare provider before changing your medication. Your healthcare provider can discuss with you the benefits of treating your condition and the risks of not treating the disease during pregnancy.
◈ I am taking voxelotor. Will it make it harder for me to get pregnant?
It is not known whether voxelotor makes it harder to get pregnant. However, having sickle cell anemia may make it harder to get pregnant.
◈ Does taking voxelotor increase the risk of miscarriage?
Miscarriage is common and can occur in any pregnancy for many reasons. There are currently no studies that have determined whether voxelotor increases the risk of miscarriage. Sickle cell anemia may increase the risk of miscarriage.
◈ Does taking voxelotor increase the risk of birth defects?
There is a 3-5% risk of birth defects in every pregnancy. This is known as background risk. Animal studies indicate that taking voxelotor during pregnancy does not increase the risk of birth defects. Currently, there are no studies confirming whether voxelotor increases the risk of birth defects in humans.
◈ Does taking voxelotor during pregnancy increase the risk of other pregnancy-related problems?
Currently, there are no human studies confirming whether voxelotor increases the risk of preterm birth (delivery before 37 weeks of gestation) or low birth weight (birth weight less than 2500 grams). Patients with sickle cell anemia may have a higher risk of preterm birth, low birth weight, and other pregnancy complications, and therefore should be closely monitored by healthcare professionals during pregnancy.
◈ Will taking voxelotor during pregnancy affect the child's future behavior or learning?
Currently, there are no studies confirming whether voxelotor causes behavioral or learning problems in children.
◈ Breastfeeding while taking voxelotor:
Currently, there are no human data on taking voxelotor while breastfeeding. The product label for voxelotor advises that breastfeeding women should not use this medication and should wait two weeks after the last dose before initiating breastfeeding. However, the benefits of using voxelotor may outweigh the potential risks. Your healthcare provider can discuss the use of voxelotor with you and the best treatment option for you. Be sure to consult your healthcare provider about all your questions regarding breastfeeding.
◈ If men take voxelotor, will it affect fertility (the ability to impregnate a partner) or increase the risk of birth defects?
Currently, no studies have explored whether voxelotor affects male fertility or increases the risk of birth defects (above background risk). One animal study suggested that male voxelotor may reduce the chances of pregnancy, but this has not been validated in humans. Additionally, sickle cell disease itself may affect male fertility. Generally, exposure to the medication by the father or sperm donor is unlikely to increase the risk of pregnancy. For more information, please see the “Paternal Exposure” information sheet on the MotherToBaby website at https://mothertobaby.org/fact-sheets/paternal-exposures-pregnancy/.

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Protein Binding
Voxelotor has an in vitro protein binding rate of 99.8%.

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In repeated-dose toxicity studies in rats (up to 1000 mg/kg/day) and dogs (up to 500 mg/kg/day) over a period of 4 weeks, Voxelotor (GBT-440; Oxbryta) did not cause significant changes in body weight, food intake, or clinical chemical parameters (ALT, AST, creatinine, BUN)[2].
-No histopathological abnormalities were observed in major organs (liver, kidney, heart, spleen) in rats and dogs receiving therapeutic doses[2]. Voxelotor (GBT-440; Oxbryta) did not cause hemolysis or damage to erythrocytes in healthy individuals at concentrations up to 500 μM[1].
-No significant drug interactions were found with CYP3A4 substrates or inhibitors in vitro[2].

[1]. GBT440 increases haemoglobin oxygen affinity, reduces sickling and prolongs RBC half-life in a murine model of sickle cell disease. Br J Haematol. 2016 Oct;175(1):141-53.

[2]. Discovery of GBT440, an Orally Bioavailable R-State Stabilizer of Sickle Cell Hemoglobin. ACS Med Chem Lett. 2017;8(3):321-326.

Voxelotor is a novel hemoglobin S-polymerase inhibitor used to treat sickle cell anemia, a genetic disorder most common in parts of the Middle East, Africa, and India. It can cause severe pain, stroke, infections, and other complications caused by blocked blood vessels. Voxelotor received accelerated approval from the U.S. Food and Drug Administration (FDA) on November 25, 2019, and is expected to become an effective treatment for the 100,000 sickle cell anemia patients in the United States and the 20 million worldwide. Developed by Global Blood Therapeutics, Voxelotor's unique mechanism of action distinguishes it from other drugs used to treat sickle cell anemia, such as hydroxyurea, L-glutamine, and crizanlizumab. The European Medicines Agency (EMA) approved voxelotor in February 2022 for the treatment of hemolytic anemia associated with sickle cell disease. Voxelotor is a hemoglobin S-polymerase inhibitor. Its mechanism of action is as a hemoglobin S polymerase inhibitor and a cytochrome P450 3A4 inhibitor. Voxelotor is an oral hemoglobin S polymerase inhibitor used to treat sickle cell disease. Rare mild to moderate elevations of serum enzymes have been observed during Voxelotor treatment, but no cases have been found associated with specific acute liver injury. Voxelotor is an orally bioavailable sickle cell hemoglobin (HbS) modulator and stabilizer with potential for treating sickle cell disease (SCD). After administration, voxelotor targets and covalently binds to the N-terminal valine residue of the HbS α-chain. This stabilizes HbS, thereby increasing its oxygen-binding affinity. The binding of voxelotor to HbS prevents HbS polymerization, reduces sickling, decreases erythrocyte damage, and prolongs erythrocyte half-life. This improves blood flow and reduces hemolytic anemia.

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Drug Indications
In the United States, voxelotor is indicated for the treatment of sickle cell disease in adults and children aged 4 years and older.
In Europe, Oxbryta is indicated for the treatment of hemolytic anemia caused by sickle cell disease (SCD) in adults and children aged 12 years and older, either as monotherapy or in combination with hydroxyurea.
Oxbryta is indicated for the treatment of hemolytic anemia caused by sickle cell disease (SCD) in adults and children aged 12 years and older, either as monotherapy or in combination with hydroxyurea.
Treatment of Sickle Cell Disease
Mechanism of Action
Sickle cell disease is characterized by the aggregation of deoxyscythemoglobin (HbS). Gene mutations that cause this disease lead to the formation of abnormal sickle-shaped red blood cells, which aggregate and block blood vessels throughout the body, causing a vascular crisis. Sickle-shaped red blood cells cannot effectively bind oxygen, thus failing to ensure a normal blood supply to organs. Voxelotor increases the oxygen affinity of hemoglobin.
Voxelotor reversibly binds to hemoglobin (Hb) by forming a covalent bond with the N-terminal valine residue of the α-chain, thereby causing allosteric modification of Hb. Voxelotor stabilizes the oxygenated Hb state and inhibits HbS polymerization by increasing the affinity of hemoglobin for oxygen.

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Pharmacodynamics
Voxelotor increases the affinity of hemoglobin (Hb) for oxygen in a dose-dependent manner. Clinical trials have shown that Voxelotor can increase hemoglobin content by up to 40%. Voxelotor may inhibit erythrocyte sickling, reduce erythrocyte deformability, and decrease whole blood viscosity.

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Voxelotor (GBT-440; Oxbryta) is an orally bioavailable HbS R-state stabilizer used to treat sickle cell anemia[1][2]
- Its mechanism of action is to bind to oxygenated HbS (R-state), stabilizing its structure and thus preventing deoxygenation-induced polymerization, erythrocyte sickling, and subsequent vascular complications[1][2]
- Voxelotor (GBT-440; Oxbryta) is the first drug approved by the FDA to directly target HbS polymerization, and was approved in 2019 for the treatment of sickle cell anemia in adults and children aged 4 years and older[2]
- In preclinical studies, Voxelotor (GBT-440; Oxbryta) showed consistent efficacy in various sickle cell disease mouse models, supporting its clinical development[1]

C19H19N3O3

337.37

337.142

1446321-46-5

71602803

Off-white to light brown solid powder

1.2±0.1 g/cm3

539.2±50.0 °C at 760 mmHg

80-82

279.9±30.1 °C

0.0±1.5 mmHg at 25°C

1.617

2.85

1

5

6

25

434

0

FWCVZAQENIZVMY-UHFFFAOYSA-N

InChI=1S/C19H19N3O3/c1-13(2)22-16(8-10-21-22)19-14(5-4-9-20-19)12-25-18-7-3-6-17(24)15(18)11-23/h3-11,13,24H,12H2,1-2H3

2-hydroxy-6-[[2-(2-propan-2-ylpyrazol-3-yl)pyridin-3-yl]methoxy]benzaldehyde

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: Please store this product in a sealed and protected environment (e.g. under nitrogen), avoid exposure to moisture.

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: ≥ 2.5 mg/mL (7.41 mM) (saturation unknown) in 5% DMSO + 40% PEG300 + 5% Tween80 + 50% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
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: ≥ 2.5 mg/mL (7.41 mM) (saturation unknown) in 5% DMSO + 95% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
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: ≥ 2.08 mg/mL (6.17 mM) 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 20.8 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL 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 4: ≥ 2.08 mg/mL (6.17 mM) 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 20.8 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 5: ≥ 2.08 mg/mL (6.17 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 20.8 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 6: 0.5 mg/mL (1.48 mM) in 1% DMSO + 99% Saline (add these co-solvents sequentially from left to right, and one by one),clear solution.
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.)