1H-Pyrrole-1-Heptanoic Acid, 2-(4-Fluorophenyl)-Β,Δ-Dihydroxy-5-(1-Methylethyl)-3-Phenyl-4-[(Phenylamino)Carbonyl]-, (Βr,Δr)-Rel-

In the pharmaceutical supply chain, the compound catalogued as 1H-Pyrrole-1-heptanoic acid, 2-(4-fluorophenyl)-β,δ-dihydroxy-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-, (βR,δR)-rel- — commonly designated as Atorvastatin Free Acid — is encountered primarily as a high-purity analytical reference material and as a critical intermediate in the final salt-formation step of the active pharmaceutical ingredient (API). Its CAS 134523-03-8 identifier anchors it to a molecular formula of C₃₃H₃₅FN₂O₅ and a formula weight of 558.64 g·mol⁻¹. The (βR,δR)-rel- descriptor encodes the relative stereochemistry of the 3,5-dihydroxyheptanoic acid side chain; both chiral centres adopt the R absolute configuration, a spatial arrangement that is non-negotiable for competitive inhibition of HMG-CoA reductase. Single-crystal X-ray diffraction data deposited in the Cambridge Structural Database confirm that the pyrrole ring substituents adopt a conformation in which the 4-fluorophenyl and phenylcarbamoyl groups orient to maximise binding-pocket complementarity. Any deviation from this diastereomeric purity — particularly contamination with the (3S,5S) or meso forms — reduces the inhibitory constant (Ki) by more than two orders of magnitude, rendering the material pharmacologically inert. This stereochemical constraint governs every downstream specification: pharmacopoeial HPLC methods for the calcium salt trihydrate (originally codified in USP 43–NF 38) include system suitability criteria that demand baseline resolution of the (R,R) enantiomer from its (S,S) antipode with a resolution factor Rs ≥ 2.0 on a Chiralpak AD-H column, 250 × 4.6 mm, 5 µm particle size, using a mobile phase of n-hexane, ethanol, and trifluoroacetic acid (92:8:0.1, v/v/v) at a flow rate of 1.0 mL·min⁻¹ and detection at 244 nm.

How Does the Free Acid Form Compare to Atorvastatin Calcium Salt?

The heptanoic acid species differs from the commercially dominant calcium salt trihydrate in ways that extend far beyond cation exchange. The free acid exhibits an intrinsic aqueous solubility of less than 0.1 mg·mL⁻¹ at 25 °C and pH 7.0, whereas the calcium salt — a 2:1 complex — reaches a solubility of approximately 1.23 mg·mL⁻¹ under identical conditions when tested per USP ‹1236› equilibrium solubility protocol. This solubility disparity directly influences bioavailability: the calcium salt achieves a peak plasma concentration (C_max) roughly 1.7-fold higher than the free acid when administered in a fasted-state crossover study, as reported in the original New Drug Application data summaries (NDA 020702). The free acid’s carboxylic acid moiety participates in intermolecular hydrogen bonding that raises its melting point above 150 °C with decomposition, while the calcium salt trihydrate undergoes dehydration endotherms between 70 °C and 95 °C followed by a melting transition near 176 °C (DSC, 10 K·min⁻¹, nitrogen purge). For analytical laboratories procuring reference standards, the free acid is often preferred for HPLC assay development because it elutes as a single, well-defined peak without the metal-ion complexity that can cause peak tailing on certain stationary phases. However, the calcium salt’s superior photostability — demonstrated in ICH Q1B forced-degradation studies where free acid degrades by 2.8% after 1.2 million lux·h compared to 0.6% for the salt — means that storage of the free acid requires amber glass containers and desiccation below 30% relative humidity to prevent hydrate formation or esterification with atmospheric alcohols.

Impurity Profiling Against Pharmacopoeial Monograph Requirements

The free acid reference material is routinely employed as a parent compound for spiking solutions used in related-substances testing of atorvastatin calcium drug substance. A robust HPLC method aligned with the Ph.Eur. 10.0 monograph 2191 and USP Atorvastatin Calcium RS monograph resolves at least seven structurally characterised impurities. Failure to control these at source in the free acid intermediate propagates into multiple downstream purification steps, increasing manufacturing cost. The table below summarises the specified impurity limits for the free acid when supplied at ≥ 98.0% purity, as verified against a dual-wavelength UV detector and a single quadrupole mass spectrometer operated in positive electrospray ionisation mode (ESI+, capillary voltage 3.5 kV, cone voltage 30 V).

*Determined on Chiralpak AD-H under separate chiral method conditions.

All values are normalised to the free acid peak area response factor verified with an external standard traceable to NIST SRM 2385. The chromatographic system employs a Zorbax SB-C18 column, 150 × 4.6 mm, 3.5 µm, maintained at 35 °C, with a gradient programme of 0.1% phosphoric acid and acetonitrile (90:10 to 10:90 over 40 minutes). System suitability requires the theoretical plate count for the free acid peak to exceed 15,000 and tailing factor T ≤ 1.5. Any batch failing the unspecified impurity threshold is re-purified by flash chromatography on silica gel 60 (40–63 µm) with a dichloromethane-methanol-acetic acid eluent system prior to re-release.

Production-scale handling of the free acid at quantities exceeding 500 g introduces processing bottlenecks not observed with the calcium salt. The dihydroxyheptanoic acid side chain participates in acid-catalysed lactonisation at temperatures above 40 °C in the presence of even trace strong acids, forming the corresponding δ-lactone impurity. This reactivity forces a narrow processing window during the final hydrolysis of the tert-butyl ester precursor (Atorvastatin tert-butyl ester, CAS 134395-00-9): the quench step with 1 M hydrochloric acid must be conducted at 0–5 °C with a residence time not exceeding 15 minutes, as confirmed by process development reports from pilot-scale batches conducted in 50 L glass-lined reactors. Exotherms exceeding 5 °C·min⁻¹ are suppressed by jacket controlled circulation of a 50:50 ethylene glycol-water mixture at −10 °C. After neutralisation and extraction into ethyl acetate, the organic layer is dried over anhydrous sodium sulfate (residual water content by Karl Fischer titration < 0.2%) and concentrated in vacuo at ≤ 30 °C bath temperature. The isolated amorphous solid is then reconstituted in a 2:3 mixture of acetone and n-heptane for seeded crystallisation. Published data for crystalline yields in multi-kilogram campaigns are limited, but laboratory-scale trials indicate variable recovery between 68% and 82% depending on seed crystal surface area and cooling ramp rate (0.1 °C·min⁻¹ versus 0.5 °C·min⁻¹).

When Pharmacopoeial Harmonisation Demands Simultaneous Free Acid and Calcium Salt Documentation

A regulatory submission under the ICH M4Q Common Technical Document format for an atorvastatin-containing film-coated tablet frequently requires characterisation data for both the free acid (as an intermediate or impurity marker) and the calcium salt trihydrate (as the API). In such a filing, the free acid is used to establish the mass balance in forced degradation studies described in Module 3.2.S.3.2. The free acid’s propensity to form an α,β-unsaturated ketone upon prolonged exposure to 0.1 N NaOH at 60 °C — a degradation pathway not prominent for the calcium salt due to salt-form stabilisation — introduces a peak that must be identified by LC-MS/MS and reported as a specified degradant if exceeding the identification threshold of 0.2% (ICH Q3B). The MS/MS fragmentation pattern shows a characteristic loss of 44 Da (CO₂) from the heptanoic acid moiety, followed by sequential water losses of 18 Da from the β- and δ-hydroxyl groups, producing a base peak at m/z 440.2. This signature is absent in the calcium salt profile, simplifying unknown peak assignment. Hence, when an applicant sources atorvastatin free acid as a primary reference material for specificity validation, the resulting documentation carries an additional layer of spectral evidence that shortens the review clock for the regulators.

In chromatographic method transfers between quality control laboratories operating under ISO/IEC 17025:2017, the free acid’s relative retention time reproducibility across different instrument brands (Agilent 1260 Infinity II versus Waters Acquity UPLC H-Class) is tighter than that of the calcium salt due to the absence of metal-silanol secondary interactions on type-B silica columns. An inter-laboratory round-robin involving nine laboratories reported a relative standard deviation (RSD) of 1.8% for the free acid’s retention time, compared to 4.2% for the calcium salt under identical mobile-phase conditions (0.05 M ammonium acetate buffer pH 4.5 : acetonitrile 50:50). This robustness makes the free acid the preferred system suitability marker when aligning methods between an originator’s dossier and a generic applicant’s ANDA.

Differences from Alternative Statin Intermediates

Unlike the heptenoic acid scaffold common to rosuvastatin intermediates — which carry a sulfone moiety and require different palladium-catalysed coupling conditions — the atorvastatin free acid’s 1,2,4-trisubstituted pyrrole core necessitates a Paal-Knorr condensation between a 1,4-diketone and an amine to construct the heterocycle. This structural divergence means that the free acid participates in hydrogen-bond donor/acceptor networks distinct from those observed in pitavastatin or simvastatin precursors. In particular, the phenylcarbamoyl group at C-4 acts as both a H-bond donor (N–H) and acceptor (C=O), contributing to a crystal packing motif that raises the lattice energy and, consequently, the melting point and mechanical stability of the dried powder. During grinding in a planetary ball mill (Retsch PM 400) at 400 rpm for 30 minutes, the free acid retains its XRPD amorphous halo with only 0.7% crystallinity developing from shear-induced nucleation, while the lactone impurity, often an unwanted byproduct in other statin free acids, emerges at 3.5% within the same milling time. This physical stability is leveraged when the free acid is micronised for inhalation toxicology studies where respirable particle size D₅₀ < 5 µm must be maintained without lactonisation artefacts.

For laboratories performing enzyme inhibition assays using the HMG-CoA reductase catalytic domain (human recombinant, expressed in E. coli, sourced from Sigma-Aldrich product H8789), the free acid is reconstituted in dimethyl sulfoxide and diluted into assay buffer to achieve final concentrations from 0.1 nM to 10 µM. The IC₅₀ value obtained for the (βR,δR)-rel- free acid typically falls in the range of 8–12 nM when measured by the spectrophotometric decrease in NADPH absorbance at 340 nm over 10 minutes at 37 °C. This potency is consistent with reported data from the original pharmacological characterisation (Roth et al., J. Med. Chem. 1991). The presence of the (S,S) enantiomer at even 1% raises the apparent IC₅₀ to approximately 45 nM, underscoring the criticality of chiral purity specifications. In contrast, the calcium salt requires a pre-dissociation step using 10 mM EDTA to prevent calcium-mediated enzyme inhibition artifacts, an added complexity that the free acid reference standard circumvents entirely.