Measuring content uniformity in low-dose tablets using near-infrared transmission analysis

Near-Infrared Transmission for Content Uniformity in Low-Dose Tablets — Application Note Summary

• Content uniformity (CU) is a critical quality attribute for compressed tablets that ensures each unit delivers the intended dose. For low-dose formulations (<2% w/w API), reliable CU measurement is challenging but essential for patient safety and regulatory compliance.
• Rapid, non-destructive analytical methods that probe the entire tablet volume improve process control and reduce reliance on solvent-based, destructive techniques such as HPLC and titration.

• Assess the capability of Fourier Transform Near-Infrared (FT-NIR) transmission spectroscopy to predict API concentration in low-dose tablets (~5 mg label claim, ~2% w/w API).
• Build and validate a calibration model that meets USP <905> content uniformity criteria and demonstrate precision and accuracy against a validated HPLC reference method.
• Evaluate practical factors affecting performance (sample presentation, repeatability) and quantify method performance metrics.

• Samples: 149 calibration standards (synthetic and production) spanning ~50% to ~150% of label claim (~2.5 mg to 7.2 mg API). Validation set: 49 independent standards across the same range.
• Calibration strategy: Synthetic samples were intentionally prepared over an extended range to improve robustness and lower prediction error for the CU-relevant range around 100% label claim.
• Spectral acquisition: Transmission-mode FT-NIR; spectral region used ~8650 cm⁻¹ to 8880 cm⁻¹.
• Preprocessing: Second derivative spectra with a Norris smoothing filter (segment length = 11, gap = 10). No scattering correction applied (constant pathlength approach).
• Chemometrics: Partial Least Squares (PLS) regression. Four latent factors selected based on principal component spectra and PRESS behavior. Outlier screening used Chauvenet’s criterion.
• Reference: Validated HPLC assay used for method traceability and calibration target values.

• Thermo Scientific Antaris FT-NIR Analyzer (transmission mode). The application note references an older Antaris model and notes the availability of the Antaris II with improved speed and performance.

• Correlation between FT-NIR predictions and HPLC reference: Pearson correlation coefficient r = 0.9816, indicating strong linear agreement.
• Calibration and prediction errors: RMSEC = 0.168 (3.3% of label claim); RMSEP = 0.149 (2.9% of label claim). PRESS plot shape supported a reasonable model without overfitting.
• Model dimensionality: Four PLS factors provided signal-related variance without introducing noise contributions.
• Repeatability: Repeated measurements (same tablet measured six times without repositioning) on five tablets produced RSD < 1.0%.
• Practical considerations: Transmission sampling interrogates the full tablet volume and therefore offers better representation of API distribution than surface-limited reflection methods. However, tablet orientation, embossing, or stamping can affect transmitted signal; consistent presentation or method accommodation is necessary to control this variability.
• Regulatory fit: The validated set satisfied USP <905> content uniformity acceptance limits for this batch.

• Non-destructive and solvent-free measurement enabling rapid throughput and reduced sample preparation compared with HPLC and titration.
• Whole-tablet interrogation via transmission improves representativeness for CU assessment, particularly for low-dose products where API heterogeneity is a concern.
• Fast implementation with minimal operator training; FT-NIR workflows can be integrated into QC laboratories and process analytical technology (PAT) frameworks.
• When combined with validated reference assays (HPLC), FT-NIR can provide robust routine screening and release testing, reducing the number of destructive assays required.

• Instrument advances (higher speed, improved detectors and optics) will further lower noise, enabling more reliable quantification at even lower API loadings.
• Enhanced chemometric strategies: improved preprocessing (adaptive scattering corrections), variable selection, and machine-learning algorithms could yield smaller prediction errors and increased robustness to tablet presentation variability.
• Integration into PAT and real-time release testing workflows for continuous manufacturing, enabling inline or at-line CU monitoring.
• Combined modality measurements (simultaneous transmission and reflection) and imaging approaches may provide richer information on coating, surface defects, and internal distribution for comprehensive quality assessment.
• Standardization of calibration transfer and validation protocols to support multicenter implementations and lifecycle management of NIR models.

• Transmission FT-NIR using the Antaris FT-NIR platform demonstrated accurate and precise prediction of API concentration in low-dose tablets, with strong agreement to a validated HPLC reference and performance metrics consistent with USP <905> acceptance criteria for the tested batch.
• Key strengths include whole-tablet interrogation, fast non-destructive analysis, and low repeatability error (RSD <1%). Attention to sample presentation and robust calibration spanning an extended concentration range are essential for reliable CU models.
• Overall, FT-NIR transmission is a practical, high-value tool for CU assessment in low-dose tablet manufacturing and QA/QC operations.

• United States Pharmacopeia. Chapter <905> Content Uniformity.
• United States Pharmacopeia. Chapter <1119> Near Infrared Spectroscopy.
• Thermo Fisher Scientific. Antaris FT-NIR Analyzer application note (AN51389, 0522) and product documentation.