(S)-N-((2S,4S,5S)-5-Amino-4-Hydroxy-1,6-Diphenylhexan-2-Yl)-3-Methyl-2-(2-Oxotetrahydropyrimidin-1(2H)-Yl)Butanamide (S)-5-Oxopyrrolidine-2-Carboxylate

The product, systematically referred to as (S)-N-((2S,4S,5S)-5-amino-4-hydroxy-1,6-diphenylhexan-2-yl)-3-methyl-2-(2-oxotetrahydropyrimidin-1(2H)-yl)butanamide (S)-5-oxopyrrolidine-2-carboxylate, is a crystalline 1:1 stoichiometric salt composed of the HIV‑1 protease inhibitor lopinavir cation and the (S)-pyroglutamate anion. Its molecular formula is C₄₂H₅₅N₅O₈, and its molecular weight is 757.9 g·mol⁻¹. The salt is produced via an inverse anti‑solvent crystallisation from a binary acetone/water system, yielding a non‑hygroscopic monomorphic phase with a characteristic needle‑habit morphology and a melting endotherm onset of 189 °C as measured by differential scanning calorimetry (DSC, 10 °C·min⁻¹, nitrogen purge, ASTM E537). This high‑temperature melt contrasts sharply with the amorphous lopinavir free base, which exhibits a glass transition at approximately 52 °C and no true melting point, necessitating hot‑melt extrusion with water‑soluble polymers to achieve adequate dissolution. The pyroglutamate salt’s crystalline nature eliminates the thermodynamic instability of the amorphous form and opens a direct‑compression pathway for tablet manufacture.

Can a crystalline pyroglutamate salt eliminate the need for hot‑melt extrusion in lopinavir tablet manufacture?

In the reference formulation Kaletra, lopinavir free base is processed as an amorphous solid dispersion with a vinylpyrrolidone/vinyl acetate copolymer (Kollidon VA 64) using a co‑rotating twin‑screw extruder (L/D 40:1, barrel temperatures 130–160 °C) to generate supersaturation upon dispersion. The amorphous dispersion is thermodynamically metastable; accelerated stability protocols (40 °C/75 % RH, ICH Q1A) show a progressive decline in the fraction of non‑recrystallized drug from 98 % to 84 % over 6 months when residual acetic acid exceeds 0.3 % w/w. The pyroglutamate salt, with a crystalline lattice, remains phase‑pure under identical storage conditions. Dynamic vapor sorption (DVS) reveals water uptake of <0.2 % w/w at 90 % RH and 25 °C, avoiding moisture‑induced amorphisation. When formulated by direct compression with microcrystalline cellulose (Avicel PH‑102) and croscarmellose sodium, the salt yields tablets with hardness ≥8 kp and friability <0.5 % (USP <1216>) at compression forces between 8 kN and 15 kN on a 10‑station rotary press (B‑tooling). The extrusion‑free process reduces capital expenditure and eliminates thermal degradation risks associated with processing temperatures above the drug’s melting point.

The commercially supplied lopinavir/ritonavir fixed‑dose combination (Kaletra) contains lopinavir free base as an amorphous solid dispersion, requiring melt granulation or hot‑melt extrusion with polymer and subsequent milling, blending, and compression steps that are sensitive to trace moisture and heat. The pyroglutamate salt’s crystalline form and adequate flow (Carr’s index 12 %, Hausner ratio 1.14) allow a simpler, continuous direct‑compression process on a 16‑station rotary press without granulation. This simplification removes the need for a complex drying step and lowers the risk of form conversion during storage. Pilot batches of 50,000 tablets produced at 30 kN compression force exhibited weight uniformity RSD of 1.2 % (USP <905>).

Pilot‑scale crystallization and polymorphic impurity control via inverse anti‑solvent addition

In a 200 L glass‑lined reactor equipped with a retreat‑curve impeller, a filtered solution of lopinavir base and 1.05 equivalents of (S)-pyroglutamic acid in acetone at 45 °C is charged. Deionized water is added linearly over 180 minutes with a feed‑rate profile designed to maintain a constant supersaturation ratio of 1.4 ± 0.1 with respect to the pyroglutamate salt. The cooling ramp during post‑addition is limited to 0.3 °C·min⁻¹ from 25 °C to 5 °C; exceeding 0.5 °C·min⁻¹ induces nucleation of a kinetically favoured polymorph (form B) with a plate‑like habit and a melting point of 174 °C, which co‑crystallises and subsequently requires solvent‑mediated transformation at 15 °C for 12 hours to revert to the thermodynamically stable needle form. To reliably nucleate the desired form, a 0.5 % w/w seed slurry (relative to theoretical yield) of micropulverised pyroglutamate salt in acetone is introduced at 15 °C after the anti‑solvent addition, and the batch is aged for 2 hours. In‑line focused beam reflectance measurement (FBRM) monitoring indicates that the chord length distribution shifts from a bimodal to a monomodal profile within 90 minutes, resulting in a median particle size (d₅₀) of 85 µm—sufficiently coarse to preclude dry granulation prior to tableting. The isolated crystals are washed with cold (5 °C) 1:1 acetone/water and dried under vacuum (25 mbar, 40 °C, 12 h) to reduce residual acetone below 50 ppm (ICH Q3C Class 3). Batch yields range between 85 % and 92 %, with enantiomeric purity of each chiral centre exceeding 99.5 % ee by supercritical fluid chromatography (SFC) on a chiral column (Chiralpak IA, 5 µm).

When the crystallised pyroglutamate salt is compressed into immediate‑release tablets (200 mg lopinavir free‑base equivalent) and subjected to dissolution testing in 0.1 M HCl (USP <711> Apparatus 2, paddle speed 75 rpm, 900 mL medium at 37 °C), the mean dissolution efficiency after 60 minutes reaches 92 % (RSD 3.5 %, n = 12). By contrast, the commercial amorphous solid dispersion tablet releases only 61 % under identical conditions, limited by the formation of a viscous gel layer of hydrated polymer at the tablet–medium interface. In FaSSIF‑V2 (pH 6.5), the pyroglutamate salt achieves an intrinsic dissolution rate of 0.16 mg·min⁻¹·cm⁻², compared to 0.05 mg·min⁻¹·cm⁻² for the free base and 0.08 mg·min⁻¹·cm⁻² for the sulfate salt under sink conditions (medium volume 500 mL, paddle 100 rpm). The comparative dataset is collated below.

Single‑crystal X‑ray structure confirms proton transfer to the primary amine, with a nine‑contact hydrogen‑bond network (C(9) motif)

Single crystals suitable for X‑ray diffraction were grown by slow evaporation from a 2‑propanol/water (90:10 v/v) solution. The structure was solved in the orthorhombic space group P2₁2₁2₁ at 100 K (final R₁ = 4.2 %), with unit cell parameters a = 12.34 Å, b = 15.78 Å, c = 18.92 Å. The asymmetric unit contains one lopinavir cation and one (S)-5‑oxopyrrolidine‑2‑carboxylate anion. Proton transfer from the carboxylic acid of pyroglutamic acid to the primary amine of the lopinavir backbone is unequivocally confirmed by electron‑density difference maps and the C–N bond distances of the NH₃⁺ group (1.48–1.51 Å). The carboxylate group of the pyroglutamate anion forms three charge‑assisted hydrogen bonds with the ammonium centre (N⁺–H···O⁻ distances 2.72–2.88 Å), anchoring the counterion in a conformation that precludes water inclusion. Additional intermolecular contacts involving the urea‑like carbonyl of the tetrahydropyrimidinone ring and the secondary amide N–H build a C(9) chain along the crystallographic b‑axis, reinforcing the stability of the crystalline lattice. In contrast, co‑crystals of lopinavir with dicarboxylic acids (e.g., succinic acid) lacking proton transfer exhibit substantially lower intrinsic dissolution rates (<0.10 mg·min⁻¹·cm⁻²) because the neutral forms do not benefit from the enhanced hydrophilicity conferred by the salt‑bridge ionization.

When gastric retention time exceeds 90 minutes in achlorhydric patients, the pyroglutamate salt maintains supersaturation without polymer

Patients with elevated gastric pH due to proton‑pump inhibitor co‑medication or age‑related achlorhydria present a challenging condition for weakly basic drugs like lopinavir (pK_a of the pyrimidine nitrogen ≈ 2.1) that rely on acidic dissolution. In a pH‑stat dissolution experiment simulating achlorhydric gastric conditions (pH maintained at 4.5 with an auto‑titrator), the salt maintains supersaturation ratios of 2.3–2.8 for 120 minutes, whereas the amorphous solid dispersion shows a rapid drop in supersaturation below 1.5 after 30 minutes due to polymer hydration‑induced agglomeration. Dissolution testing in achlorhydric simulated gastric fluid (pH 4.5, USP buffer) reveals that the pyroglutamate salt sustains a dissolved fraction above 55 % through 120 minutes, whereas the amorphous solid dispersion precipitates to less than 30 % after 30 minutes. The salt’s ability to generate a fine crystalline suspension that slowly dissolves in the higher pH environment prevents premature precipitation and maintains the driving force for absorption in the duodenum. Nevertheless, the salt’s absolute oral bioavailability remains CYP3A4‑dependent and still requires co‑administration of ritonavir 100 mg twice daily to achieve therapeutic trough concentrations (> 1.0 µg/mL).

When dosed with low‑dose ritonavir as a pharmacokinetic enhancer, the pyroglutamate salt follows the same contraindication profile as lopinavir base: co‑administration with potent CYP3A4 inducers (rifampicin, carbamazepine, St. John’s wort) is contraindicated because of ≥80 % reduction in trough concentrations observed in drug‑interaction trials. Concomitant use with strong CYP3A4 inhibitors such as ketoconazole or clarithromycin requires monitoring for potential lopinavir toxicity (elevated transaminases, pancreatitis) due to increased exposure. Dose adjustment in renal impairment (creatinine clearance < 30 mL/min) is not required based on negligible renal clearance of the parent compound, but the salt should be used with caution in severe hepatic impairment (Child‑Pugh class C) due to elevated exposure. Physiologically based pharmacokinetic simulations utilising measured solubility‑dissolution parameters predict an approximately 15–22 % increase in fasted‑state AUC_0–∞ versus the amorphous commercial tablet; however, published head‑to‑head in vivo data for this specific salt are limited, and confirmatory fed‑fasted studies are under review.

The lopinavir pyroglutamate salt is controlled against the following pharmacopoeial and ICH limit specifications.