ABSTRACT Intracellular concentrations of isoniazid and rifabutin resulting from administration of inhalable microparticles of these drugs to phorbol-differentiated THP-1 cells and the pharmacokinetics and biodistribution of these drugs upon inhalation of microparticles or intravenous administration of free drugs to mice were investigated. In cultured cells, both microparticles and dissolved drugs established peak concentrations of isoniazid (∼1.4 and 1.1 μg/10 ⁶ cells) and rifabutin (∼2 μg/ml and ∼1.4 μg/10 ⁶ cells) within 10 min. Microparticles maintained the intracellular concentration of isoniazid for 24 h and rifabutin for 96 h, whereas dissolved drugs did not. The following pharmacokinetic parameters were calculated using WinNonlin from samples obtained after inhalation using an in-house apparatus (figures in parentheses refer to parameters obtained after intravenous administration of an equivalent amount, i.e., 100 μg of either drug, to parallel groups): isoniazid, serum half-life ( t _1/2 ) = 18.63 ± 5.89 h (3.91 ± 1.06 h), maximum concentration in serum ( C _max ) = 2.37 ± 0.23 μg·ml ⁻¹ (3.24 ± 0.57 μg·ml ⁻¹ ), area under the concentration-time curve from 0 to 24 h (AUC _0-24 ) = 55.34 ± 13.72 μg/ml ⁻¹ h ⁻¹ (16.64 ± 1.80 μg/ml ⁻¹ h ⁻¹ ), and clearance (CL) = 63.90 ± 13.32 ml·h ⁻¹ (4.43 ± 1.85 ml·h ⁻¹ ); rifabutin, t _1/2 = 119.49 ± 29.62 h (20.18 ± 4.02 h), C _max = 1.59 ± 0.01 μg·ml ⁻¹ (3.47 ± 0.33 μg·ml ⁻¹ ), AUC _0-96 = 109.35 ± 14.78 μg/ml ⁻¹ h ⁻¹ (90.82 ± 7.46 μg/ml ⁻¹ h ⁻¹ ), and CL = 11.68 ± 7.00 ml·h ⁻¹ (1.03 ± 0.11 ml·h ⁻¹ ). Drug targeting to the lungs in general and alveolar macrophages in particular was observed. It was concluded that inhaled microparticles can reduce dose frequency and improve the pharmacologic index of the drug combination.