Contribution of excipient-borne nitrite to nitrosamine drug substance-related impurity formation in secondary amine-containing solid dosage forms under accelerated storage

Nitrosamine drug substance-related impurities (NDSRIs) have emerged as a critical quality concern following worldwide product recalls that revealed N-nitrosodimethylamine (NDMA) and related compounds in marketed pharmaceuticals at levels exceeding internationally accepted thresholds. While previous investigations focused on process-related sources of nitrosating agents such as contaminated solvents and recycled intermediates the contribution of pharmaceutical excipients as reservoirs of residual nitrite capable of driving NDSRI formation during storage has received comparatively little systematic attention. This research quantified the residual nitrite content in five commonly used pharmaceutical excipients (croscarmellose sodium, pregelatinised starch, povidone K30, microcrystalline cellulose, and magnesium stearate) sourced from four different commercial suppliers, and then measured the formation of NDMA-equivalent nitrosamine in model tablet formulations containing either ranitidine hydrochloride or metformin hydrochloride as representative secondary amine-containing active pharmaceutical ingredients. Tablets were manufactured by direct compression and stored under ICH-recommended accelerated (40 °C / 75% relative humidity) and long-term (25 °C / 60% RH) conditions for 12 weeks, with sampling at 0, 2, 4, 8, and 12 weeks. Nitrite content was determined by a validated ion-chromatography method compliant with pharmacopoeial methodology [14], and NDMA-equivalent levels were quantified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) with a limit of quantification of 0.5 ng per tablet. Residual nitrite varied more than 30-fold across excipient types and suppliers, ranging from 0.08 µg/g in magnesium stearate to 2.74 µg/g in a high-nitrite lot of croscarmellose sodium. In ranitidine tablets formulated with high-nitrite croscarmellose sodium, NDMA-equivalent levels reached a median of 83 ng per tablet after 12 weeks at 40 °C / 75% RH, more than three times the ICH M7 acceptable intake limit of 26.5 ng per day. The same formulation prepared with a low-nitrite grade of the same excipient produced a median of 26 ng per tablet at the threshold margin. Long-term storage conditions generated substantially lower but still measurable NDSRI levels (median 35 ng per tablet for the high-nitrite croscarmellose, 10 ng for the low-nitrite grade). Metformin tablets showed a parallel but attenuated pattern, consistent with the lower reactivity of the metformin secondary amine group toward nitrosation compared with the ranitidine dimethylamino moiety. Multivariate regression of all five predictors (excipient nitrite content, storage temperature, API identity, relative humidity, and excipient type) yielded an adjusted R² of 0.87 (P < 0.001), with excipient nitrite content and storage temperature emerging as the two dominant contributors by standardised coefficient magnitude. These findings establish excipient-borne nitrite as a quantitatively important and controllable root cause of NDSRI formation during storage and support the adoption of nitrite-specification limits for excipients used in secondary amine-containing dosage forms.