Roller-compacted tablet cores at 4.0 wt% API load

Dry granulation by roller compaction is selected for crisaborole tablet cores when direct compression is rejected because the micronized API lot is specified at d90 ≤ 25 µm by USP <429> laser diffraction and the untapped compressibility index remains above 35% by USP <1174>. The API is pre-blended with pregelatinized starch, microcrystalline cellulose, croscarmellose sodium, and colloidal silicon dioxide after all excipients are passed through a 500 µm sieve and pre-dried at 40°C until loss on drying is below 2.0% by USP <731>. Because no compendial oral tablet monograph for crisaborole is published, the addition ratio is calculated from the target unit strength divided by uncoated core mass: a 10 mg strength in a 250 mg core corresponds to 4.0 wt%, and a 30 mg strength in a 400 mg core corresponds to 7.5 wt%. Ribbons are compacted on a Gerteis Mini-Pactor using smooth rolls with a 1.0–2.0 mm gap and hydraulic pressure adjusted to maintain ribbon solid fraction between 0.55 and 0.65. The milled granulation is required to reach d50 between 180 µm and 250 µm and is compressed on a 22-station Korsch XL 400 press with Euro-D tooling at 8–18 kN compression force. Resulting cores are held to hardness 60–120 N, friability below 1.0% by USP <1216>, and content uniformity meeting USP <905>. In-process controls align with 21 CFR 211.110, equipment qualification with 21 CFR 211.65, and release stability evaluation with 21 CFR 211.166. Terminal product type is a film-coated tablet, with aqueous Opadry II coat applied to a 3.0–4.0% weight gain. If ribbon solid fraction drifts below 0.55, the subsequent milling operation generates fines above 40% of granulation mass, causing sticking and weight variation at compression; therefore, ribbon porosity is monitored by near-infrared densitometry immediately downstream of the compactor. The release dissolution test uses USP <711> Apparatus II at 50 rpm in 900 mL of 0.1 N HCl with sampling at 15 min, 30 min, 45 min, and 60 min. Magnesium stearate is limited to ≤ 1.0 wt% because excess hydrophobic lubricant can prolong disintegration and retard dissolution of a low-solubility API.

Why Does Low Aqueous Solubility Constrain 30 mg Capsule Content Uniformity?

Low aqueous solubility of crisaborole constrains direct capsule filling because micronized drug particles tend to segregate in low-fill-weight blends, producing weight variation and unacceptable content uniformity under USP <905>. A spray-dried solid dispersion is therefore introduced as the oral capsule intermediate. The API and vinylpyrrolidone-vinyl acetate copolymer are dissolved in a mixed acetone-methanol solvent at an API-to-polymer ratio of 20:80 w/w, and the solution is spray-dried at inlet temperature 90–110°C, outlet temperature 45–60°C, and feedstock solids content between 8–12 wt%. Residual solvent levels must satisfy ICH Q3C, with acetone not exceeding 5000 ppm and methanol not exceeding 3000 ppm by USP <467>. The resulting amorphous dispersion is milled to d50 ≤ 100 µm and filled into size 0 or size 1 hydroxypropyl methylcellulose capsule shells; a 300 mg fill containing 30 mg crisaborole corresponds to 10 wt% API in the final capsule fill. Capsule filling is conducted on a Bosch GKF 2000 machine at 50,000–80,000 capsules/h, with in-process weight checks under 21 CFR 211.110 and final release testing by USP <905> and USP <711>. Because published data for crisaborole in this specific spray-dried configuration are limited, amorphicity and crystallinity are confirmed by modulated differential scanning calorimetry and X-ray powder diffraction after storage for 6 months at 25°C/60% RH. The spray-dryer outlet relative humidity is maintained below 20% to reduce amorphous phase separation and moisture-induced recrystallization. Terminal product type is a hard HPMC capsule packaged in cold-form aluminum-PVC blisters with a desiccant pouch. Batch failure mode observed during scale-up of similar low-solubility solid dispersions is capsule shell embrittlement when residual solvent is inadequately dried; therefore, the residual solvent limit is treated as a release criterion rather than an in-process monitoring point.

When Lyophilized Cake Collapse Occurs in Boron-Containing Formulations

A lyophilized powder for injection is used when terminal sterilization is not feasible and the aqueous solubility of crisaborole is insufficient for a stable ready-to-use liquid presentation. The bulk solution is prepared at an API addition ratio of 5.0 mg/mL (0.5% w/v) in Water for Injection, with pH adjusted to 7.0–7.5 using 0.1 N sodium hydroxide. A 2.0 mL fill into a 10 mL Type I glass vial yields a 10 mg lyophilized cake per vial. The solution is sterile-filtered through a 0.22 µm PVDF membrane and filled under EU GMP Annex 1 Grade A conditions with ISO 14644-1 class 5 air. Lyophilization uses shelf temperature ramped from -40°C to +25°C at chamber pressure 0.1 mbar, with primary drying duration adjusted by pirani-versus-capacitance manometry; endpoint is confirmed when chamber pressure and product temperature converge and residual moisture is below 1.0% by Karl Fischer titration. Cake collapse occurs when shelf temperature exceeds the collapse temperature of the formulation; for crisaborole, the collapse temperature must be determined by freeze-dry microscopy before scale-up because published data for this specific configuration are limited. Terminal product type is a lyophilized powder for injection that must be reconstituted with 10 mL Sterile Water for Injection. Release and stability tests include USP <1>, USP <788> particulate matter at thresholds of ≥ 10 µm and ≥ 25 µm, USP <85> bacterial endotoxins, and USP <790> visible particulates. Incoming excipient controls follow 21 CFR 211.84. Mannitol is retained as a bulking agent only if a free-API assay by HPLC after reconstitution remains within 95–105% of label claim; if reversible boronate ester formation between the benzoxaborole ring and mannitol reduces free drug concentration, glycine is substituted as a nonsaccharide bulking agent.

Granule Size Distribution Shifts in High-Shear Wet Granulation

High-shear wet granulation is selected for low-dose oral granules when a sachet presentation must provide dose flexibility and when dry blending cannot achieve acceptable content uniformity. The addition ratio in the dry granulation charge is calculated as API mass divided by final sachet fill mass: a 5 mg dose in a 1000 mg granule fill equals 0.5 wt%, and a 10 mg dose in the same fill mass equals 1.0 wt%. The dry powders are charged into a Glatt vertical granulator and granulated at impeller speed 300 rpm and chopper speed 1500 rpm, with purified water added at 8–12% of dry powder mass over 3–5 min. Wet mass is discharged through a 500 µm screen and dried in a fluid-bed dryer with inlet air temperature 50–60°C until loss on drying is below 2.0% by USP <731>. The dried granulate is sieved through a 500 µm screen; fines below 150 µm are limited to ≤ 30% of total mass because excess fines cause fill-weight variation on sachet-filling lines and increase dusting during reconstitution. Granule d50 shifts from 180–250 µm toward the fine fraction when chopper speed exceeds 1800 rpm for more than 5 min, which is a defined process failure because it increases sachet weight variation. Terminal product type is a granule for oral suspension filled into paper-foil-polyethylene sachets, with each sachet reconstituted in 20 mL of water before administration. Compliance standards for the granule intermediate are ICH Q3D elemental impurities tested per USP <232>/<233> and ICH Q3C residual solvents tested per USP <467>, with in-process controls under 21 CFR 211.110. Sorbitol and mannitol are excluded from the formulation unless equilibrium solubility studies demonstrate negligible free-API reduction, because the benzoxaborole ring can reversibly bind vicinal diols; lactose monohydrate and microcrystalline cellulose are preferred diluents.

Wet media milling of crisaborole to a nanosuspension is used when an oral liquid platform must address dissolution-rate-limited drug release and when dose titration requires a liquid dosage form. The API is pre-dispersed in purified water containing hydroxypropylmethylcellulose E3 at 0.5–1.0% w/v and polysorbate 80 at 0.05–0.10% w/v; the slurry is milled in a Netzsch DeltaVita 50 L stirred media mill loaded with 0.3 mm yttrium-stabilized zirconia beads at tip speed 6–10 m/s. Incoming API is pre-milled to d90 ≤ 25 µm before wet milling, because coarse feed particles increase milling time and bead wear. Milling is continued for 120–240 min with recirculation until photon correlation spectroscopy shows d50 200–300 nm and d90 ≤ 500 nm. The final suspension addition ratio is 1.0–2.0 wt%, corresponding to 10–20 mg/mL crisaborole. Terminal product type is an oral suspension filled into amber polyethylene terephthalate bottles with child-resistant closures. Release specifications include particle size by USP <429>, pH by USP <791>, and antimicrobial effectiveness by USP <51>. In-process controls follow 21 CFR 211.110, and the mill circuit is maintained at 20–25°C. If the suspension exceeds 35°C during milling, Ostwald ripening accelerates and d90 drifts above 500 nm within 48 h. Milling yield after 240 min is insufficient if bead fill volume is below 70%; low bead fill prolongs milling time and produces a multimodal size distribution that cannot meet the d90 release criterion.

For single-dose vials that must remain in liquid form, crisaborole is dissolved in a sterile co-solvent system composed of 20% v/v polyethylene glycol 300 and 10% v/v ethanol in Water for Injection, with pH adjusted to 7.0–7.4 using 0.1 N sodium hydroxide. The API addition ratio is maintained at 1.0 mg/mL (0.1% w/v) for a 10 mg/10 mL single-dose vial, because published intravenous tolerability data for higher concentrations of crisaborole are limited. The solution is sparged with nitrogen to dissolved oxygen below 2 ppm, passed through a 0.22 µm PVDF sterilizing filter, and aseptically filled into Type I borosilicate glass vials under EU GMP Annex 1 Grade A conditions. Terminal product type is a ready-to-use injectable solution, not a lyophilized intermediate. Release testing includes USP <788> particulate matter at ≥ 10 µm and ≥ 25 µm, USP <85> bacterial endotoxins, and USP <791> pH. Filter integrity is verified by pre-use and post-use bubble point in accordance with EU GMP Annex 1; a post-use bubble point below the manufacturer minimum is a batch rejection criterion. The formulation boundary is fixed at an upper ethanol concentration of 10% v/v and a maximum polyethylene glycol 300 concentration of 20% v/v to avoid precipitation upon dilution in 0.9% w/v sodium chloride injection and to remain within USP <1> injection vehicle limits. Incoming drug substance lots containing residual elemental impurities above the ICH Q3D parenteral limit are rejected under 21 CFR 211.84; elemental impurity carryover can otherwise pass through sterile filtration and remain in the finished vial.