In solid-phase peptide synthesis of C18-diacid modified GLP-1 receptor agonist intermediates, octadecanedioic acid mono-tert-butyl ester is introduced after Fmoc removal from the ε-amino group of a lysine residue or from a resin-bound linker amine. Resin swelling in a DMF/dichloromethane mixture at 70:30 v/v for 30–45 min at 20–25 °C precedes Fmoc deprotection with 20% piperidine in DMF, giving substitution values of 0.30–0.60 mmol/g. The protected C18 diacid is activated either as an N-hydroxysuccinimide ester or in situ with ethyl 2-cyano-2-(hydroxyimino)acetate and diisopropylcarbodiimide at a molar input of 1.10–1.30 equivalents relative to free resin-bound amine. Coupling is held at 20–25 °C for 2–4 h; a negative Kaiser test after double coupling indicates residual amine below 0.05% of initial loading. Capping with acetic anhydride/pyridine at 1:1 v/v is applied to unreacted amine sites, and the N-terminal Fmoc group is removed under the same piperidine conditions before peptide cleavage. Cleavage from 2-chlorotrityl chloride or Rink amide resin is performed with trifluoroacetic acid/triisopropylsilane/water at 95:2.5:2.5 v/v for 2 h at 20–25 °C, after which the crude peptide is precipitated with cold methyl tert-butyl ether and vacuum filtered. Preparative reversed-phase high-performance liquid chromatography using a C18 column and a mobile phase of 0.1% trifluoroacetic acid in water/acetonitrile achieves target purity of ≥99.0% by area normalization. On 100 mmol pilot-scale resin lots, retention of solvent moisture below 0.10% w/w by Karl Fischer reduces coupling yield variation to less than 3% batch-to-batch. Compliance for this route is anchored to ICH Q7 section 7.4 for recovery of materials and solvents, 21 CFR 210.3(b) for in-process control definitions, and USP <467> for residual solvent verification. Terminal products are mono-lipidated peptide APIs for subcutaneous injection and oral tablet intermediates in which the octadecanedioic acid moiety provides reversible albumin binding.

When the Protected C18 Diacid Replaces Activated Lipid Esters in Solution-Phase Conjugation

Multi-kilogram solution-phase campaigns for injectable peptide APIs use the mono-tert-butyl-protected octadecanedioic acid as a protected lipid anchor that is converted to a mixed anhydride with pivaloyl chloride in tetrahydrofuran at -5 °C to 5 °C. Condensation with the ε-amino group of a lysine residue on a protected peptide fragment occurs in an aqueous tetrahydrofuran system at pH 7.8–8.4. The stoichiometric set point is 1.03–1.15 mol of protected diacid per mole of amino component; exceeding 1.20 mol generates tert-butyl ester impurities that co-elute with the mono-lipidated product on C18 HPLC within 0.8 min of the target retention time. Temperature control at ±2 °C during mixed anhydride formation limits symmetric anhydride formation and preserves tert-butyl ester integrity. Reaction progress is monitored by in-process HPLC every 30 min; free peptide residual content is acceptable at ≤0.10% w/w. Quenching with 0.5 M citrate buffer at pH 3.5 arrests coupling and precipitates unreacted protected diacid. The crude product is extracted into 2-methyltetrahydrofuran, washed twice with 10% sodium chloride, precipitated from n-heptane, and vacuum dried at 35–40 °C for 12–18 h. Production-scale batch records for a 20 L stirred reactor show that nitrogen blanketing below 2% oxygen in headspace and visual confirmation of a single liquid phase before coupling avoid yield losses above 5%. Compliance is established under ICH Q11 section 3.2, 21 CFR 211.110, and USP <905> for finished dosage uniformity. Terminal product types include lyophilized powder for subcutaneous injection and sterile solution API for prefilled syringe assembly.

Before tablet compression begins, roller compaction of oral tablet blends containing a C18-diacid peptide API prepared from the tert-butyl-protected octadecanedioic acid intermediate requires control of residual protected ester impurity. API entering the tablet line is specified at ≤0.10% w/w residual mono-tert-butyl ester by HPLC, because the tert-butyl group can hydrolyze to isobutylene and tert-butyl alcohol under acidic tablet film-coating or gastric fluid conditions, producing unidentified peaks in stability-indicating methods. The tablet core is formulated with API loading of 0.5–4.0% w/w, lactose monohydrate 60–80% w/w, microcrystalline cellulose 10–25% w/w, croscarmellose sodium 2.0–5.0% w/w, and sodium stearyl fumarate 0.5–1.0% w/w. API particle size is controlled at D50 35–65 µm and D90 ≤120 µm to prevent segregation; the final blend is verified for content uniformity at 10 sampling points per batch. Compression on a rotary tablet press with 9 mm round tooling at 8–15 kN force and 25–40 rpm turret speed produces tablets of 100–400 mg total mass and 4–7 kP hardness. Friability is held below 0.8% after 100 rotations per USP <1216>, disintegration per USP <701> occurs in less than 15 min in water at 37 °C, and dissolution per USP <711> uses USP apparatus 2 at 50 rpm in 900 mL of 0.1 M hydrochloric acid for the first 30 min, followed by phosphate buffer pH 6.8. Regulatory compliance is under 21 CFR 211.165, ICH Q3D, and USP <905>. Terminal products are immediate-release oral tablets for administration of C18-diacid peptide APIs.

What Limits Residual Protected Diacid Carryover in Oral Capsule and Granule Filling?

Residual protected diacid carryover in oral capsule and granule operations is controlled through final API purification after deprotection, with a released API specification for octadecanedioic acid mono-tert-butyl ester of ≤0.15% w/w. Granulation for encapsulation uses a high-shear mixer at impeller speed 250–350 rpm and chopper 1200–1800 rpm for 180–240 s wet massing; granule drying at 40 °C inlet air in a fluid bed dryer reduces loss on drying to ≤1.5% w/w. The dried granulate is screened through a 0.8 mm sieve and blended with crospovidone 2.0–4.0% w/w, colloidal silicon dioxide 0.1–0.3% w/w, and magnesium stearate 0.25–0.5% w/w. API load in the granule fraction is 0.3–2.5% w/w; the remainder consists of mannitol and microcrystalline cellulose in a 1:1 ratio. HPMC capsule filling on a Bosch GKF 720 at 70,000–90,000 capsules/h requires granule bulk density 0.55–0.65 g/mL, tapped density 0.68–0.80 g/mL, and Hausner ratio 1.15–1.35 to maintain weight RSD ≤2.0%. Capsule shell moisture is conditioned at 25 °C/50% RH for 24–48 h before filling; in-process weight checks are performed at 15 min intervals. Compliance for finished capsules is demonstrated by USP <905>, USP <711>, Ph. Eur. 2.9.5, and 21 CFR 211.165. Terminal forms are HPMC capsules and granule-filled sachets for oral lipidated peptide APIs; capsule strengths are adjusted through granule API content rather than fill volume changes.

For subcutaneous injectable formulations built around C18-diacid peptide APIs manufactured from the mono-tert-butyl-protected octadecanedioic acid intermediate, solutions are filled as sterile solutions at 1.0–5.0 mg/mL in 10 mM phosphate buffer at pH 7.4 with 8.0% trehalose or sucrose as lyoprotectant. Terminal sterile filtration through 0.22 µm polyvinylidene fluoride or polyethersulfone membranes at 20–25 °C uses a differential pressure not exceeding 0.8 bar; pre-filtration bioburden is required to be ≤10 CFU/100 mL before the final membrane. Filter compatibility testing under 21 CFR 211.67 and ISO 13408-2 ensures that the C18-diacid peptide API does not reduce membrane bubble point below 3.2 bar for PVDF. The lyophilization cycle includes freezing at -45 °C for 180 min, primary drying at -20 °C and 0.20 mbar for 20–30 h, and secondary drying at 30 °C for 6–10 h; chamber shelf temperature uniformity is maintained within ±1.5 °C. Production-scale lyophilizers with 20 m² shelf area show edge-vial moisture variability below 0.2% w/w when the cooling rate is controlled at 0.5 °C/min. Residual moisture by coulometric Karl Fischer titration is specified at ≤1.0% w/w. Subvisible particulate testing per USP <788> and Ph. Eur. 2.9.19 requires counts for particles ≥10 µm not more than 6000 per container and for particles ≥25 µm not more than 600 per container. Endotoxin is controlled at ≤0.5 EU/mg for injectable API, and visible particulates are assessed under USP <790> after 8 h of reconstitution at 25 °C. Terminal product types include lyophilized single-dose vials, prefilled syringes, and dual-chamber cartridges for subcutaneously administered C18-diacid peptide APIs; preservative-free presentations require aseptic filling in an ISO 14644-1 Class 5 environment with continuous viable monitoring.

Residual Solvent Control Points for tert-Butyl Deprotection in Oral and Injectable API Batches

Residual solvent control after tert-butyl deprotection of octadecanedioic acid mono-tert-butyl ester becomes a batch-release parameter because the deprotection cocktail contains trifluoroacetic acid, triisopropylsilane, and water, while the workup uses methyl tert-butyl ether, dichloromethane, and N,N-dimethylformamide. The final API is re-precipitated from methyl tert-butyl ether and n-heptane at 1:3 v/v at 0–5 °C for 2–4 h, filtered, and vacuum dried at 35–40 °C for 12–24 h under ≤5 mbar. Residual protected ester is specified at ≤0.10% w/w for oral tablet API and ≤0.15% w/w for capsule-grade API by HPLC; residual palladium and heavy metals are controlled under ICH Q3D if catalytic hydrogenation is part of the route. Headspace gas chromatography with flame ionization detection uses a DB-624 capillary column (30 m × 0.32 mm × 1.8 µm) with injection port at 200 °C, oven ramp from 40 °C to 220 °C, and headspace equilibration at 80 °C for 30 min. The acceptance limits in the table below are aligned with ICH Q3C and USP <467>. Published data for residual trifluoroacetic acid in long-chain monoester peptide APIs is limited; therefore, in-house release specifications are set at ≤100 ppm until process capability data justify a wider limit under an ICH Q3C risk assessment. The same solvent control framework applies to oral tablet, capsule, granule, and injectable finished dosage forms, with injectable API requiring a lower water content limit of ≤0.5% w/w if the product is lyophilized from a non-aqueous solvent system.