Inhaled pentamidine. An overview of its pharmacological properties and a review of its therapeutic use in Pneumocystis carinii pneumonia

Pentamidine is an aromatic diamidine derivative which has become one of the standard therapies for Pneumocystis carinii pneumonia (PCP), particularly in patients with acquired immunodeficiency syndrome (AIDS). However, with parenteral administration of the drug there is a high risk of toxicity. Inhaled pentamidine produces much higher concentrations of drug on the bronchoalveolar surface with minimal systemic absorption. It has been used successfully for the treatment of PCP in AIDS patients, but its most valuable contribution has been as prophylaxis in AIDS patients at high risk of developing PCP. In prospective controlled studies there has been greater than 80% reduction in relapse rate with pentamidine. The reduction in relapse rate among patients who have experienced one previous episode of PCP has been 50 to 100% compared with historical control groups, over a follow-up period averaging about 6 months. Significant systemic adverse effects to inhaled pentamidine are rare. Respiratory effects associated with inhalation are common but usually controllable without treatment discontinuation. The ideal particle size for even distribution of pentamidine throughout the lung is considered to be 1 to 2 microns. Jet nebulisers such as the 'Respirgard II' system produce a mass median aerodynamic diameter (MMAD) of particles in this range. Ultrasonic nebulisers produce larger particles. The implication from this difference is that while ultrasonic nebulisers may have poorer alveolar distribution and the incidence of local side effects (common with all formulations) may be higher, total drug delivery may be more efficient allowing effective PCP prophylaxis with lower dosages (120 mg vs 300 mg monthly). However, there are no data available comparing the efficacies and tolerabilities of the different formulations of inhaled pentamidine. Nevertheless, inhaled pentamidine would seem poised to become routine prophylaxis in patients with AIDS or AIDS-related complex at risk of developing PCP.