This research explored in-vitro and in-vivo performance of three salbutamol metered dose inhalers (MDIs): Ventolin Evohaler (Evo), Airomir (Airo) and Salamol. In the in-vitro studies, critical quality attributes of the MDI using an Andersen cascade impactor (ACI) were examined and included measurement of fine particle dose (FPD) and total delivered dose (TDD). Bioequivalence studies were conducted in humans using the urinary pharmacokinetic method. Post-inhalation urinary excretion of salbutamol in the first 0.5 hour (lung deposition, USAL0.5) and over 24 hours (total systemic bioavailability, USAL24) were compared to determine the bioequivalence of the MDIs. The spacers recommended for use with these inhalers were also studied, and charcoal block studies were performed to assess the extent of USAL0.5. The three MDIs had FPD (μg) of 78, 91 and 89, respectively; the latter pair was equivalent. Their USAL0.5 (6, 7 & 7 μg) was however not bioequivalent. These MDIs delivered equivalent dose (177, 174 & 180 μg) which reflected on their USAL24 (101, 84 & 97 μg). Nevertheless, USAL24 was inequivalent between Evo and Airo. The FPD of Evo with Volumatic (VOL), AeroChamber Plus (AERO) and Able spacer was 78, 68 and 74 μg, respectively. The AERO treatment method was not equivalent to the MDI while VOL and Able were equivalent between them. Spacer USAL0.5 (16, 15 & 14 μg) was not bioequivalent to the MDI but to each other. The spacer in-vitro TDD (95, 85 & 92 μg) was inequivalent to the MDI treatment method. In contrast, their USAL24 was bioequivalent (97, 85 & 90 μg). The FPD of Airomir with AERO (95 μg) was in-vitro equivalent while USAL0.5 (15 μg) of this treatment method was bio-inequivalent to the MDI alone. On the contrary, the TDD (110 μg) and USAL24 (84 μg) of AERO were respectively in-vitro inequivalent and bioequivalent to the MDI alone. The FPD (μg) of Salamol MDI alone and with VOL (84) and AERO (86) as well as between the spacers was equivalent. However, the USAL0.5 of the MDI was not bioequivalent to spacers (20 and 18 μg) despite being equivalent between the spacers. In contrast, the respective TDD (103 and 95 μg) of spacer treatment methods were in-vitro inequivalent to the MDI alone albeit having bioequivalent USAL24 (86 and 87 μg). The variations in the in-vitro performance of the three MDIs are most likely due to differences in their formulations and designs. As the performance metrics of the MDI influence lung deposition, substituting one MDI with another can have clinical implications. Although the spacers reduced in-vitro TDD of the MDI to about half, their use increased lung deposition by over two folds, the magnitude of which varied with the MDI and spacer type. Despite significant decrease in dose delivery, the total systemic bioavailability with the spacers was similar to that with the MDI alone. This systemic bioequivalence is more likely due to greater USAL0.5 with the spacers. The results of the charcoal block studies reinforced this outcome. The present study is unique as it used a clinically relevant salbutamol MDI dose (two puffs), assessed results for equivalence and analysed ACI deposition data further as stage groups. The deposition on adjacent ACI stages were grouped together as coarse, fine and extra-fine particle masses to identify their more likely deposition sites in the human respiratory tract. Moreover, this thesis describes highly sensitive and novel HPLC and SPE methods, developed and validated to quantify salbutamol in urinary and aqueous matrices. As the clinical effects of MDIs are related to their lung deposition, the current work emphasizes the importance of spacer use. Nevertheless, differences in dose delivery between spacers may have clinical consequences. Hence, only the specific spacer recommended for use with the MDI should be used. / World Federation, Stanmore, London and Sadaat Welfare Foundation, Bradford, West Yorkshire