Acta Pharm. 68 (2018) 31-46
Original research paper
A headspace-gas chromatography method for isopropanol determination in warfarin
sodium products as a measure of drug crystallinity
ZIYAUR RAHMAN, SOHAIL AKHTAR, AKHTAR SIDDIQUI,
ANTHONY B. CIAVARELLA, AGNES NGUYENPHO, PATRICK J. FAUSTINO and MANSOOR A. KHAN
1 Division of
Product Quality and Research, Center for Drug Evaluation and Research, Food and
Drug Administration, Maryland, USA
2 Irma
Lerma Rangel College of Pharmacy, Texas
A&M Health Science Center, Texas A&M University, Reynolds Medical
Building, Suite 159 College Station, Texas 77843-1114, USA
Accepted October 3, 2017
Published online January 22,
2018
Coumadin® and several generic
products of warfarin sodium (WS) contain the
crystalline form (clathrate) in which WS and isopropanol (IPA) are associated in a 2:1 molar ratio. IPA
is critical in maintaining the WS crystalline structure. Physicochemical
properties of the drug and drug product may change
when the crystalline drug transforms to amorphous form. A headspace-gas
chromatography (HS-GC) method was developed and validated for IPA determination
in the WS drug product. n-propanol (NPA) was used as internal standard and the
method was validated for specificity, system suitability, linearity, accuracy,
precision, range, limits of detection and quantification, and robustness. The
method was specific, with good resolution between IPA and NPA peaks.
Chromatographic parameters (retention time, IPA/NPA area ratio, tailing factor,
theoretical plates, USP symmetry, capacity factor, selectivity and resolution)
were consistent over three days of validation. The analytical method was linear
from 2–200 µg mL–1 (0.1–10 % IPA present in the drug product). LOD and LOQ were 0.1 and 2 µg
mL–1, respectively.
Accuracy at low (2 µg mL–1) and high (200 µg mL–1)
IPA concentrations of the calibration curve was 103.3–113.3 and
98.9–102.2 % of the nominal value, resp. The validated method was precise, as
indicated by the RSD value of less than 2 % at three concentration levels of
the calibration curve. The method reported here was utilized to determine
accurately and precisely the IPA content in in-house formulations and commercial
products. In summary, IPA determination by HS-GC provides an indirect measure
of WS crystallinity in the drug product. Nevertheless,
it should be confirmed by another analytical method since IPA from the drug
substance is not distinguishable from IPA that may be present outside the drug
crystals in a dosage form when prepared by wet granulation with IPA.
Keywords: warfarin crystallinity, isopropanol,
headspace-gas chromatography