Efaproxiral (RSR13) is a synthetic allosteric modifier of hemoglobin (Hb) and a bezafibrate analog used for brain metastases originating from breast cancer. Efaproxiral has been shown to bind reversibly to hemoglobin, stabilizing the deoxyhemoglobin tetramer conformation to reduce its affinity for oxygen. Efaproxiral plus oxygen breathing reduces the radiobiological hypoxic fraction of EMT6 tumors from the value of 24% found in both air-breathing and oxygen-breathing mice to 9% and improves the response of the tumors to radiation. It also has the potential to be used for the treatment of depression, traumatic brain injury, ischemia, stroke, myocardial infarction, diabetes, hypoxia, sickle cell disease, hypercholesterolemia and as a radio sensitiser.

Physicochemical Properties

Molecular Formula	C20H23NO4
Molecular Weight	341.4
Exact Mass	341.163
CAS #	131179-95-8
Related CAS #	Efaproxiral sodium;170787-99-2;Efaproxiral-d6;1246815-16-6
PubChem CID	122335
Appearance	White to off-white solid powder
Density	1.202g/cm3
Boiling Point	554.5ºC at 760mmHg
Flash Point	289.2ºC
Vapour Pressure	3.93E-13mmHg at 25°C
Index of Refraction	1.597
LogP	3.799
Hydrogen Bond Donor Count	2
Hydrogen Bond Acceptor Count	4
Rotatable Bond Count	6
Heavy Atom Count	25
Complexity	458
Defined Atom Stereocenter Count	0
SMILES	CC(C)(OC1=CC=C(CC(NC2=CC(C)=CC(C)=C2)=O)C=C1)C(O)=O
InChi Key	BNFRJXLZYUTIII-UHFFFAOYSA-N
InChi Code	InChI=1S/C20H23NO4/c1-13-9-14(2)11-16(10-13)21-18(22)12-15-5-7-17(8-6-15)25-20(3,4)19(23)24/h5-11H,12H2,1-4H3,(H,21,22)(H,23,24)
Chemical Name	2-[4-[2-[(3,5-dimethylphenyl)amino]-2-oxoethyl]phenoxy]-2-methylpropanoic acid.
Synonyms	RSR13 RSR-13 RSR 13 NSC-722758 NSC722758 NSC 722758 Efaproxyn
HS Tariff Code	2934.99.9001
Storage	Powder-20°C 3 years 4°C 2 years In solvent -80°C 6 months -20°C 1 month
Shipping Condition	Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

Biological Activity

ln Vitro	In the central water cavity of Hb, efaproxiral only binds to two symmetrically linked locations [2]. When serum albumin solution is present, epsilon easily passes through the red blood cell membrane [2]. In the presence of anion channel blockers (DIDS), efaproxiral entry into red blood cells is not impeded [2].
ln Vivo	Efaproxiral (150 mg/kg, intraperitoneal injection) can enhance tumor oxygenation and boost the inhibitory effect of radiation on tumor growth within 5 days of treatment [3]. Efaproxiral lowers hemoglobin-oxygen binding affinity, stimulates hemoglobin to release oxygen to adjacent tissues, and may increase tumor pO(2)[4]
Animal Protocol	Animal/Disease Models: Female C3H/HEJ mice (18-20 g), radiation-induced fibrosarcoma tumors ( RIF-1) cell xenograft [3] Doses: 150 mg/kg Route of Administration: intraperitoneal (ip) injection; prior to X-ray irradiation (4 Gy/day) for 5 days Experimental Results: Significant increase in tumor oxygenation over 5 days 8.4 to 43.4 mmHg, with the greatest increase occurring 22-31 minutes after treatment.
References	[1]. Stea B, et al. Efaproxiral red blood cell concentration predicts efficacy in patients with brain metastases. Br J Cancer. 2006 Jun 19;94(12):1777-1784. [2]. Abraham DJ, et al. Allosteric modifiers of hemoglobin: 2-[4-[[(3,5-disubstituted anilino)carbonyl]methyl]phenoxy]-2-methylpropionic acid derivatives that lower the oxygen affinity of hemoglobin in red cell suspensions, in whole blood, and in vivo in rats. [3]. Hou H, et al. The effects of Efaproxyn (efaproxiral) on subcutaneous RIF-1 tumor oxygenation and enhancement of radiotherapy-mediated inhibition of tumor growth in mice. Radiat Res. 2007 Aug;168(2):218-25. [4]. Hou H, et al. Increased oxygenation of intracranial tumors by efaproxyn (efaproxiral), an allosteric hemoglobin modifier: In vivo EPR oximetry study. Int J Radiat Oncol Biol Phys. 2005 Apr 1;61(5):1503-9.
Additional Infomation	Efaproxiral is a synthetic small molecule with radiosensitizing activity. Efaproxiral increases oxygen levels in hypoxic tumor tissues by binding non-covalently to the hemoglobin tetramer and decreasing hemoglobin-oxygen binding affinity. Increasing tumor oxygenation reduces tumor radioresistance. (NCI04) See also: Efaproxiral Sodium (annotation moved to).

Solubility Data

Solubility (In Vitro)

DMSO : ≥ 150 mg/mL (~439.37 mM)

Solubility (In Vivo)

Solubility in Formulation 1: ≥ 2.08 mg/mL (6.09 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 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. Solubility in Formulation 2: ≥ 2.08 mg/mL (6.09 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 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. Solubility in Formulation 3: ≥ 2.08 mg/mL (6.09 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. Solubility in Formulation 4: 27.5 mg/mL (80.55 mM) in 50% PEG300 50% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution; with ultrasonication. Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.  (Please use freshly prepared in vivo formulations for optimal results.)

Preparing Stock Solutions		1 mg	5 mg	10 mg
	1 mM	2.9291 mL	14.6456 mL	29.2912 mL
	5 mM	0.5858 mL	2.9291 mL	5.8582 mL
	10 mM	0.2929 mL	1.4646 mL	2.9291 mL

*Note: Please select an appropriate solvent for the preparation of stock solution based on your experiment needs. For most products, DMSO can be used for preparing stock solutions (e.g. 5 mM, 10 mM, or 20 mM concentration); some products with high aqueous solubility may be dissolved in water directly. Solubility information is available at the above Solubility Data section. Once the stock solution is prepared, aliquot it to routine usage volumes and store at -20°C or -80°C. Avoid repeated freeze and thaw cycles.