Validated HPLC assay detects trace impurity in montelukast synthesis

A laboratory-validated reversed-phase high-performance liquid chromatography method can quantify residual 7-chloroquinaldine in the alkene aldehyde intermediate used to manufacture montelukast sodium. Researchers developed the assay with ultraviolet detection at 230 nm and demonstrated a linear analytical response over 0.49–3.72 ppm. The reported limit of quantification was 0.4932 ppm, while the limit of detection was 0.0493 ppm.

Results from prepared standards, spiked intermediate and montelukast sodium tablet material support the assay for impurity monitoring, although the study did not test whether its use changes manufacturing yields, final-product quality or clinical outcomes. Alkene aldehyde, formally identified as 3-(2-(7-chloroquinolin-2-yl)-(E)-vinyl)benzaldehyde, is an intermediate in the synthesis of montelukast sodium. The manufacturing route begins with 7-chloroquinaldine and isophthalaldehyde as starting materials for the alkene aldehyde.

Residual unreacted 7-chloroquinaldine can therefore remain in the intermediate. If it is not controlled, it may persist through subsequent stages and carry over into the final drug substance. The research addresses an analytical gap rather than a clinical question: the authors stated that no reverse-phase HPLC method had previously been reported for assessing 7-chloroquinaldine during production of the alkene aldehyde intermediate.

The study was a method-development and validation exercise conducted in accordance with International Council for Harmonisation principles. The investigators evaluated specificity, linearity, detection and quantification limits, repeatability, intermediate precision, accuracy and recovery, robustness, and solution stability. Method development and validation used a Shimadzu LC 2010 HPLC system with LabSolutions software, while a Waters Alliance e2695 system equipped with a photodiode-array detector was also used.

The evidence comes from laboratory-prepared standards, spiked alkene aldehyde, and montelukast sodium tablets obtained from a local pharmacy. It was not a clinical investigation, a comparison of manufacturing processes, or a trial conducted on full-scale production batches. To choose a detection wavelength, the team scanned separate 7-chloroquinaldine and alkene aldehyde solutions, each prepared at 250 ppm, across the ultraviolet range of 200–400 nm.

Both compounds showed significant absorbance at 230 nm, which was selected for analysis. Researchers also evaluated C8, phenyl and C18 stationary phases. Among them, an XBridge C18 column provided satisfactory retention, peak shape and resolution for the impurity.

Formic acid was selected as the aqueous modifier after investigations involving ammonium acetate, ammonium bicarbonate, water–acetonitrile, water–methanol and formic-acid systems. The final chromatographic conditions were chosen according to resolution, peak shape, sensitivity, robustness and total analysis time. The finished method used a 150 × 4.6 mm C18 column with 5-micrometre particles, a flow rate of 1.0 mL per minute, a 10-microlitre injection volume and a 35-minute gradient run.

The aqueous buffer contained 0.075% formic acid and was adjusted to pH 3.0 with dilute ammonia before sonication. Buffer and acetonitrile were combined at 50:50 to form mobile phase A and at 10:90 to form mobile phase B. The column was maintained at ambient temperature, the detector operated at 230 nm, and the mobile phases were filtered through 0.45-micrometre nylon membranes. Under these conditions, 7-chloroquinaldine was separated from the alkene aldehyde and other tested matrix components without interfering peaks at the relevant retention time.

Specificity was evaluated with a blank solution, the intermediate, the impurity and montelukast sodium tablet powder spiked at 2.5 ppm with 7-chloroquinaldine and alkene aldehyde. The chromatograms showed baseline separation of 7-chloroquinaldine from the intermediate and montelukast sodium. Six replicate injections of the impurity produced a relative standard deviation below the study’s 10% threshold, supporting consistent peak response.

Spectral purity was also examined using the photodiode-array data. The reported purity angle for 7-chloroquinaldine was 2.433, below the corresponding purity threshold of 3.052. In combination with the observed chromatographic separation, these measurements supported the selectivity of the method under the tested conditions.

Calibration was performed at six levels ranging from approximately 0.49 ppm, corresponding to the quantification limit, to 3.72 ppm, or 150% of the target concentration. Each level was prepared and analysed in triplicate. Regression analysis yielded a correlation coefficient of 0.9992 and an R-squared value of 0.9983.

The slope was 61,762, and the intercept was 2,415.09 ± 3,348.29. Detection and quantification limits were established through a signal-to-noise approach, with the detection target defined at a ratio of 3:1 and the quantification target at 10:1. The resulting limit of detection was 0.0493 ppm and the limit of quantification was 0.4932 ppm.

At the quantification limit, six replicate measurements had a relative standard deviation of 5.76%, within the study’s stated acceptance criterion of no more than 10%. Overall repeatability was reported at a relative standard deviation of 0.04%, while intermediate precision had a relative standard deviation of 4.57%. Recovery experiments covered the quantification limit and the 50%, 100%, 120% and 150% concentration levels.

Recoveries ranged from 96.80% to 100%, with relative standard deviations from 0.26% to 0.58%. These laboratory measurements showed close agreement between added and measured quantities over the tested levels. The authors interpreted the combined validation results as showing that the assay could support routine quantitative analysis of residual 7-chloroquinaldine in the alkene aldehyde.

Robustness was assessed by varying the mobile-phase flow rate from its nominal 1.0 mL per minute to 0.9 and 1.1 mL per minute and by changing detection wavelength from 230 nm to 228 and 232 nm. For each condition, six system-suitability injections were made along with injections of the alkene aldehyde and a 100% spiked 7-chloroquinaldine sample. Recovery values and relative standard deviations remained within the predefined acceptance ranges.

Separate 7-chloroquinaldine and alkene aldehyde solutions stored on a benchtop at room temperature were monitored for 24 hours. Their peak-area relative standard deviations were 0.13% and 0.60%, respectively, indicating stability of the prepared solutions over that period. The validated method’s main practical value is its ability to measure a residual starting-material impurity during production of an intermediate used for montelukast sodium, before that intermediate proceeds through further reactions.

The work does not demonstrate that implementing the method automatically improves reaction yields, removes impurities from the final drug substance or alters clinical safety. It also does not report interlaboratory reproducibility, solution stability beyond 24 hours, or measurements from full-scale manufacturing batches. Within those limits, the study supplies a sensitive and reproducible laboratory assay for a quality-control problem upstream of montelukast sodium drug-substance production.

Subject of Research: Validation of an RP-HPLC assay for residual 7-chloroquinaldine in a montelukast sodium manufacturing intermediate

Article Title: Development and validation of an RP-HPLC method for the quantification of residual 7-chloroquinaldine impurity in alkene aldehyde intermediate of montelukast sodium

Article References: Patel, J. N., Joshi, K. K., Samnani, P., & Deokar, A. R. (2026). Development and validation of an RP-HPLC method for the quantification of residual 7-chloroquinaldine impurity in alkene aldehyde intermediate of montelukast sodium. Discover Chemistry, 3(1), Article 588. https://doi.org/10.1007/s44371-026-01028-2

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DOI: 10.1007/s44371-026-01028-2

Keywords: RP-HPLC, 7-chloroquinaldine, Montelukast sodium, pharmaceutical impurities, analytical validation, drug intermediates, process control, HPLC-UV, C18 chromatography, quality control, Development, validation