New HPLC assay for urinary salbutamol concentrations in samples collected post-inhalation

A new reversed phase high performance liquid chromatography (HPLC) method with fluorescence detection and two solid phase extraction (SPE) methods have been developed, optimised and validated for determining salbutamol in human urine after an inhalation. SPE methodology for unchanged salbutamol (USAL) and salbutamol plus its metabolites (USALMET) concentrations in urine has been developed using terbutaline as the internal standard. Confirm HCX cartridges were used for USAL and Oasis HLB for USALMET. Calibration lines of salbutamol urine standards were linear over the range 25-300 microg/L with mean (RSD) r(2) values of 0.9983 (0.06%) for USAL and 0.9976 (0.202%) for USALMET. The HPLC method was accurate (mean bias -0.40% for USAL and 0.46% for USALMET) and precise (mean RSD 5.0% for USAL and 2.90% for USALMET). The calculated LOD and LOQ for salbutamol using a 1 mL urine sample were 4.0 and 12.12 microg/L for USAL, and 4.80 and 14.56 microg/L for USALMET, respectively. The mean (RSD) SPE recoveries of salbutamol were 90.82% (2.32%) for USAL and 91.54% (2.96%) for USALMET. Both HPLC and SPE methods were applied to quantify unchanged and metabolised salbutamol excreted in urine after the inhalation of 200 microg salbutamol from metered dose inhalers (MDIs) by 14 healthy volunteers. Charcoal slurries were also ingested to prevent gastro-intestinal absorption. Urine samples were collected at 30 min post-inhalation and then pooled for the next 24h. All urine concentrations were within the sensitive portion of the assay. The volunteer study revealed that following inhalation from an MDI about 20% of the nominal dose is deposited into the lungs and 46% is delivered to the systemic circulation. The results confirm the application, sensitivity, reliability and robustness of the HPLC and SPE methods for urinary pharmacokinetic studies after salbutamol inhalations using therapeutic doses.