Pharmaceutical-grade (1S,2S,3S,5S)-5-(2-amino-6-(benzyloxy)-9H-purin-9-yl)-3-(benzyloxy)-2-(benzyloxymethyl)cyclopentanol, Pharma Grade API for Tablet / Capsule / Granule / Injection, Oral & Injectable, is a stereochemically defined, benzyl-protected purine cyclopentanol analogue supplied for oral and injectable manufacturing. The relative molecular mass is 551.65 g/mol, and the molecular formula is C32H33N5O4. The three benzyl-derived substituents—O6-benzyl on the purine base, C3 benzyloxy, and C2 benzyloxymethyl—create a lipophilic solid with reduced aqueous wettability relative to unprotected guanosine analogues. Commercial model designations are vendor-specific and should be confirmed on the certificate of analysis; the systematic chemical name remains the primary identity. The substance is not described by a public pharmacopoeial monograph; therefore specification limits are justified under ICH Q6A and the manufacturer’s process validation data. Published data for this specific protected configuration are limited with respect to aqueous solubility, intrinsic dissolution rate, log P, and solid-state stability; these parameters should be generated according to OECD 105 or shake-flask methods before formulation. The material is manufactured under cGMP and requires control of chiral identity because the (1S,2S,3S,5S) stereochemistry is essential to downstream deprotection or pharmacological performance.

How Does Benzyl Protection Alter the Physicochemical Profile Relative to Unprotected Guanine Analogues?

Compared with unprotected entecavir (CAS 209216-23-9), the subject compound lacks the free 6-oxo group and the free primary and secondary hydroxyl groups. The O6-benzyl substitution suppresses lactam-lactim tautomerism of the purine base, shifts the ultraviolet absorption envelope, and removes a key hydrogen-bonding donor. The C3 benzyloxy and C2 benzyloxymethyl groups further increase steric volume around the cyclopentane scaffold, which may reduce recognition by nucleoside transporters if the compound is evaluated in cellular assays. Direct pharmacological activity should not be assumed; the compound is commonly used as a synthetic intermediate or a lipophilic prodrug candidate. Differences from chlorinated purine intermediates are meaningful because the 2-amino-6-benzyloxy substitution avoids acid-labile 6-chloro intermediates and permits processing under mild basic or neutral conditions. Deprotection to the free guanosine analogue is generally achieved by catalytic hydrogenolysis over palladium catalysts, not by aqueous acid hydrolysis. Consequently the protected form is more compatible with non-aqueous granulation and organic co-solvent systems than with highly polar aqueous hydrogen-bonding excipients.

Other protected guanosine analogues may carry methoxymethyl, trityl, or tert-butyldimethylsilyl groups instead of benzyl groups. The benzyl group is useful because it remains stable under many nucleoside coupling conditions but can be removed by catalytic hydrogenolysis under near-neutral conditions. This contrasts with trityl groups, which require acidic deprotection and may promote cyclopentanol dehydration. The C2 benzyloxymethyl substituent also provides a differentiated deprotection sequence: the primary hydroxyl is not released until the benzyl group is removed, which can reduce undesired oxidation during storage. These structural differences affect the impurity profile and should be reflected in the related substances method.

Batch release testing normally includes liquid chromatographic assay, related substances, water content by Karl Fischer, residual solvents, elemental impurities, and particle size distribution. A suitable HPLC method may use a C18 column with ultraviolet detection at 254 nm; the total related substances acceptance threshold is frequently set at ≤0.10% area, but the value must be derived from toxicological qualification or batch data and not assumed from final drug product monographs. Identification is confirmed by infrared absorption spectrophotometry per USP <197> and retention time agreement with a qualified reference standard. Chiral purity should be controlled by a method capable of detecting the enantiomer or diastereomer; this is especially important because the stereochemical configuration at C1, C2, C3, and C5 defines the product. The acceptance criterion for individual stereoisomeric impurities is often set at ≤0.10%, but published data for this compound are limited.

Method validation for assay and related substances should include forced degradation under acid, base, oxidative, thermal, and photolytic conditions. The main degradation products are expected to arise from benzyl ether cleavage, oxidation of the secondary alcohol, or inversion at C1. The HPLC method should separate the deprotected guanosine analogue, the debenzylated impurity, and the enantiomer if present. A system suitability solution containing the API and critical impurities should demonstrate resolution of at least 2.0 between adjacent peaks, with tailing factor not more than 2.0. These values are typical for pharmacopoeial methods and should be verified for this compound.

Particle Size, Polymorph, and Crystallinity Specifications for Oral and Injectable Processing

Control of particle size distribution becomes critical when the API is formulated as a tablet or capsule because the benzyl-protected solid has low aqueous solubility. If dissolution rate limits bioavailability, micronisation or jet milling may be required; laser diffraction per USP <429> should be used to define D10, D50, and D90. A typical early formulation lead with D90 ≤30 µm is common for poorly water-soluble APIs, but the value is not product-specific and should be linked to a dissolution method. Surface energy changes after milling may cause agglomeration and poor flow; storage below 40% RH at 25 °C reduces moisture uptake only if the API is non-hygroscopic, and forced degradation data are required. Polymorph identity is monitored by X-ray powder diffraction per USP <941> and differential scanning calorimetry with a heating rate of 10 °C/min. If more than one crystalline form is found, the specification must identify the form because dissolution, melting point, and filtration behaviour may vary. For injectable powders, particle size also affects suspension syringeability and filter load, so the same D90 criterion may require tightening when the API is suspended rather than dissolved.

Jet milling with a classifier may achieve particle sizes suitable for dissolution-limited oral products, but the process temperature should remain below the onset of melting or glass transition to avoid amorphous conversion. High amorphous content can increase hygroscopicity and accelerate benzyl ether hydrolysis. Milled material should be re-tested for particle size and moisture because increased surface area elevates water uptake.

When Direct Compression Replaces Aqueous Granulation for Tablet and Capsule Dose Forms

Direct compression is appropriate only when the API and excipients demonstrate sufficient flow and compressibility. The protected molecule may present plate-like or needle-like morphology, which reduces flow and can lead to weight variability on high-speed rotary tablet presses. In such cases roller compaction or slugging is introduced before final compression. If aqueous granulation is used, the susceptibility of benzyl ether groups to acid-catalysed hydrolysis should be evaluated; neutral aqueous binder solutions are generally low-risk, but strong acid or high temperature should be avoided. Solvent granulation using anhydrous ethanol or isopropanol is often preferred because it restricts water exposure and evaporates without leaving a high moisture content. Blend uniformity should be tested according to ASTM E2810-19 or USP <905>, and tablet content uniformity should meet an acceptance value of ≤15.0. Capsule filling on dosator or tamping pin machines requires powder density, particle size, and Carr index data to maintain fill weight accuracy. Tablet and capsule formulations containing the protected API should avoid dibasic calcium phosphate dihydrate if moisture-promoted degradation is observed in excipient compatibility studies.

For injectable presentations, the API is dissolved or suspended in a vehicle that is compatible with the benzyl-protected structure. The choice of aqueous versus non-aqueous solvent depends on measured solubility in water for injection, pH, and co-solvent tolerance. Terminal sterilisation by moist heat at 121 °C for 15 min may be applicable only after forced degradation studies demonstrate no assay loss, no chiral inversion, and no visible colour change. Aseptic filtration is used where the API is thermolabile; filter compatibility testing is required because lipophilic compounds may adsorb to polyvinylidene fluoride or nylon membranes. Bacterial endotoxin control follows Ph.Eur. 2.6.14 and USP <85>; the limit is calculated from the maximum single dose and route of administration using the K/M formula. If benzyl alcohol is a potential degradation product, the total benzyl alcohol exposure must be compared with the safety threshold for the intended patient population because benzyl alcohol is a known neurotoxicity risk in paediatric and high-dose injectable settings. Subvisible particulate matter should comply with USP <788> or Ph.Eur. 2.9.19 where the final product is injected.

Residual solvents and elemental impurities are controlled at the route-dependent boundary

Residual solvent testing is performed by headspace gas chromatography per USP <467>. The most likely solvents are methanol, ethanol, isopropanol, ethyl acetate, and dichloromethane; each must comply with the applicable ICH Q3C Class 2 or Class 3 limit. For an injectable grade, residual solvent levels should be re-evaluated because the daily solvent exposure may be higher relative to oral solid dose. Elemental impurities are controlled by ICH Q3D option selection and analysed by ICP-MS per USP <233>. Palladium, iron, and zinc are common catalytic or processing metals; if palladium-catalysed hydrogenolysis is used downstream, palladium content is often controlled at ≤10 ppm for oral grades and lower for injectable grades, but the limit must be justified by route-specific permitted daily exposure. Batch data should be trended because elemental impurity levels can shift with catalyst life, reactor alloy wear, and filter media.

Scale-up observations on production-scale equipment indicate that batch-to-batch variability is driven primarily by crystallisation temperature, seeding, and drying endpoint. If the drying step is terminated too early, residual solvent or water levels may not meet injectable specifications; if the product is dried below an acceptable particle size, electrostatic charging can reduce powder flow and increase sticking during tablet compression. Therefore the API manufacturer should define a drying curve and not rely solely on a fixed drying time. High-shear wet granulation with torque monitoring can identify lot-to-lot binder demand differences caused by surface area variation. Twin-screw processing with defined L/D ratio is an option when continuous manufacturing is used, but the feed rate and specific mechanical energy should be matched to the API’s flow behaviour.

Operational boundaries include pre-drying at elevated humidity and incompatibility with strong acids or oxidising agents. If the API is stored above 60% RH, moisture uptake may shift assay or water content; pre-drying at 40–50 °C may be required before weighing or blending. Strong oxidising agents should be avoided because they can degrade the purine ring and cleave benzyl ethers. Aqueous granulation below pH 3 should not be used unless forced degradation data demonstrate stability at the intended processing time and temperature.

What Limits the Use of This Protected Compound as a Direct Substitute for Deprotected Guanine Antiviral APIs?

The protected benzyl groups change solubility, dissolution, and metabolic lability; therefore the compound should not be considered equivalent to the free guanosine analogue in a tablet or injection formula. In vivo hydrolysis or hydrogenolysis of the benzyl groups is not guaranteed to occur at the required rate. If the intended route is injectable, benzyl alcohol release is a potential concern because benzyl alcohol has established safety limits and its total exposure must be calculated. The product is positioned as a controlled source of a stereochemically defined purine cyclopentanol intermediate or a lipophilic prodrug candidate rather than a direct substitution for an existing monograph drug substance. When formulation studies compare the protected API with other protected or unprotected analogues, forced degradation under ICH Q1A conditions and excipient compatibility per ICH Q8 should be included.

Specification transfer between the API supplier and the dosage-form manufacturer should include the full impurity profile, residual solvent data, elemental impurity data, particle size method, and a qualified reference chromatogram. The absence of a public monograph means that analytical method validation should follow ICH Q2(R1) for accuracy, precision, specificity, linearity, and range. Tablet, capsule, and granule formulations require the same API quality attributes, but injectable dosage forms impose additional requirements for endotoxin, sterility, particle load, and solvent purity. Release for oral solid dosage should not be accepted on the basis of injectable test data alone because the route-specific risk profile differs.