(R)-4-methyl-1,3,2-dioxathiolane 2,2-dioxide, CAS 1609-05-5, is a pharma-grade cyclic sulfate active pharmaceutical ingredient supplied as a white to off-white crystalline powder. The molecular formula is C₃H₆O₄S and the relative molecular mass is 138.14 g/mol. The molecule is a five-membered cyclic sulfate ester carrying a methyl substituent at the 4-position and an S,S-dioxide; the (R) configuration is controlled by chiral HPLC against the corresponding reference standard. The substance is not listed as a compendial monograph in a major pharmacopeia; the registration specification is therefore structured according to ICH Q6A and the analytical procedures are validated under ICH Q2(R1). Four physical grades are released under the manufacturing site model designations RDX-PSD-20, RDX-PSD-80, RDX-PSD-150, and RDX-PSD-250. RDX-PSD-20 is a micronized injectable grade with laser-diffraction D90 ≤ 20 µm. RDX-PSD-80 is intended for oral granulation and has D90 ≤ 80 µm. RDX-PSD-150 is intended for capsule filling and has D90 ≤ 150 µm. RDX-PSD-250 is a direct-compression grade with D90 ≤ 250 µm. The four grades share the same chemical purity specification but differ in particle size distribution, tapped density, water limit, and microbial/endotoxin controls. The product is stored at 2–8 °C in double low-density polyethylene liners inside a sealed high-density polyethylene drum under nitrogen, with a retest period of 24 months for unopened containers.

What Specification Limits Are Applied to Injectable and Oral Grades?

Release testing follows a single pharmacopeial registration specification with additional sterility and endotoxin tests for the injectable grade. Identity is confirmed by infrared absorption spectrophotometry and by chiral HPLC retention time against the (R)-reference standard. Assay is performed by stability-indicating HPLC with acceptance criteria of 99.0–101.0% w/w on the anhydrous, solvent-free basis. Chiral purity is controlled at ≥ 99.5% enantiomeric excess, with the (S)-enantiomer limited to ≤ 0.5% w/w. Related substances are controlled as total impurities ≤ 0.5% w/w, unspecified impurities ≤ 0.10% w/w, and any specified impurity ≤ 0.15% w/w. Water content by Karl Fischer titration is ≤ 0.5% w/w for oral grades and ≤ 0.2% w/w for RDX-PSD-20. Residue on ignition is ≤ 0.1% w/w. Elemental impurities are tested according to ICH Q3D and comply with oral and parenteral concentration limits; the injectable grade is additionally controlled for bacterial endotoxins by USP <85> at <0.050 EU/mg, and for particulate matter by USP <788> after reconstitution. Residual solvents are controlled by USP <467> Option 1 and meet ICH Q3C limits; Class 1 solvents are below the limit of detection, and Class 2 solvents are below the parenteral concentration limits for the injectable grade.

The tablet and capsule grades are typically processed by direct compression or dry granulation to avoid exposing the cyclic sulfate to aqueous granulation media. RDX-PSD-250 is passed through a 500 µm conical sieve before charging into a 600 L bin blender; blend uniformity is verified at 10 sample points with acceptance criteria of 90.0–110.0% label claim and RSD ≤ 5.0% according to USP <905>. Tablet compression on a rotary press equipped with 8 mm round or capsule-shaped tooling typically uses 8–15 kN compression force, and the resulting tablets are coated only with non-aqueous film-coating systems because aqueous coating can initiate surface hydrolysis. Capsule filling with RDX-PSD-150 is performed on a dosator-type capsule filler; the tapped density is specified at 0.45–0.65 g/mL to maintain acceptable weight variation. For granulation, RDX-PSD-80 is blended with anhydrous dibasic calcium phosphate and pregelatinized starch, then granulated with an anhydrous ethanolic povidone K-30 solution in a high-shear granulator at 2.0–3.0 m/s impeller tip speed. Drying is performed in a fluid-bed dryer at inlet air temperature 45–55 °C to a final loss-on-drying ≤ 0.5% w/w. A typical tablet core formulation contains the API at 10.0% w/w, anhydrous dibasic calcium phosphate at 69.0% w/w, microcrystalline cellulose at 20.0% w/w, and sodium stearyl fumarate at 1.0% w/w; blend mixing is conducted at 25 rpm for 15 min in a bin blender.

Moisture Uptake, Hydrolytic Ring Opening, and Excipient Compatibility Boundaries

The 1,3,2-dioxathiolane 2,2-dioxide ring is moisture-sensitive. Uncontrolled exposure to relative humidity above 40% RH at 25 °C produces ring-opened mono- and diol-sulfate derivatives that fail the related-substances limit. Formulation and handling are therefore specified at ≤ 40% RH; pre-drying of excipients is required when their moisture content exceeds 2.0% w/w. Aqueous binders, maltodextrin with high water activity, and primary amines are incompatible because the amine can open the cyclic sulfate ester. Magnesium stearate is acceptable at 0.25–0.75% w/w in direct compression when total water is ≤ 0.5% w/w. For injectable compounding, the API is dissolved in anhydrous dimethylacetamide or propylene glycol; water-for-injection is added only after filtration, and the final solution is used immediately. Cellulose ester filters are avoided because of moisture-mediated degradation and pore-surface interaction; polyethersulfone or polytetrafluoroethylene filters are used. Sterile filtration is performed with a 0.22 µm filter under EU GMP Annex 1 Grade A conditions after pre-filtration through a 0.45 µm filter. Aseptic fill is preferred because terminal steam sterilization at 121 °C produces rapid hydrolysis in aqueous solution; published data for the specific pH-rate profile for this grade is limited, and formulation-specific forced degradation under ICH Q1A(R2) is required before any heat sterilization claim. The cyclic sulfate ring is more sensitive to residual free water than conventional neutral APIs, and scale-up from laboratory to production batches should include moisture mapping at each unit operation.

When the (R)-Configuration Is Compared with Racemic or (S)-Propylene Sulfate

The (R)-configured material is differentiated from the racemate and the (S)-enantiomer by chiral purity, solid-state behavior, and regulatory documentation. The racemic mixture is not interchangeable with the (R)-API in applications where the second enantiomer is regarded as an undesired related substance. Under ICH Q6A Decision Tree #2, chiral identity and enantiomeric impurity tests are included for release and stability. The (S)-enantiomer is limited to ≤ 0.5% w/w in the (R)-API. Differences from non-pharma-grade propylene sulfate include control of inorganic sulfate, sulfur trioxide, residual epoxide, and chlorinated solvent impurities; industrial material is often shipped without aqueous hydrolysis limits, whereas this pharma grade includes a sulfate ion limit of ≤ 0.2% w/w and a total unknown impurity limit of ≤ 0.10% w/w. The two physical model grades differ in particle size and flow properties: the injectable grade has D90 ≤ 20 µm and tapped density 0.25–0.45 g/mL, while the direct-compression grade has D90 ≤ 250 µm and tapped density 0.55–0.75 g/mL. The comparative table below summarizes the grade-dependent release attributes.

Release of RDX-PSD-20 for injectable use requires the additional tests of sterility by USP <71>, bacterial endotoxins by USP <85> with acceptance criterion <0.050 EU/mg, particulate matter after reconstitution by USP <788>, and container-closure integrity. The API is double-bagged inside a Grade C cleanroom and the outer drum is wiped down before transfer to Grade A/B for sterile compounding. Batch-to-batch variance in particle size distribution is controlled through jet milling with nitrogen at feed pressure 2–4 bar; higher feed pressures above 6 bar have been observed on production equipment to increase surface electrostatic charge and reduce flowability. The API is not mixed with polyethylene glycols above 60 °C because the combination can accelerate sulfate ester hydrolysis. For oral solid dosage forms, the final blend is stored no longer than 72 h at ≤ 40% RH before compression or encapsulation. Compliance with 21 CFR 210/211 and ICH Q7 is maintained through a quality management system that covers cleanroom environmental monitoring, solvent recovery validation, and annual product quality review. The material is an electrophilic API and requires dedicated or validated cleaning procedures to avoid cross-contamination with nucleophilic drug substances.