2: Mechanistic Modeling of Pulmonary Availability of Inhaled Drugs versus Critical Product Attributes

This work is the second in a series of publications outlining the fundamental principles and proposed design of a biopharmaceutics classifications system for inhaled drugs and drug products (the iBCS). Here, a mechanistic computer-based model has been used to explore the sensitivity of the primary biopharmaceutics functional output parameters: (i) pulmonary fraction dose absorbed (F_abs) and (ii) drug half-life in lumen (t_1/2) to biopharmaceutics-relevant input attributes including dose number (Do) and effective permeability (P_eff). Results show the nonlinear sensitivity of primary functional outputs to variations in these attributes. Drugs with Do < 1 and P_eff > 1 × 10^-6 cm/s show rapid (t_1/2 < 20 min) and complete (F_abs > 85%) absorption from lung lumen into lung tissue. At Do > 1, dissolution becomes a critical drug product attribute and F_abs becomes dependent on regional lung deposition. The input attributes used here, Do and P_eff, thus enabled the classification of inhaled drugs into parameter spaces with distinctly different biopharmaceutic risks. The implications of these findings with respect to the design of an inhalation-based biopharmaceutics classification system (iBCS) and to the need for experimental methodologies to classify drugs need to be further explored.

Principles and Framework of an Inhalation-Based Biopharmaceutics Classification System

For oral drugs, the formulator and discovery chemist have a tool available to them that can be used to navigate the risks associated with the selection and development of immediate release oral drugs and drug products. This tool is the biopharmaceutics classification system (giBCS). Unfortunately, no such classification system exists for inhaled drugs. The perspective outlined in this manuscript provides the foundational principles and framework for a classification system for inhaled drugs. The proposed classification system, an inhalation-based biopharmaceutics classification system (iBCS), is based on fundamental biopharmaceutics principles adapted to an inhalation route of administration framework. It is envisioned that a classification system for orally inhaled drugs will facilitate an understanding of the technical challenges associated with the development of new chemical entities and their associated new drug products (device and drug formulation combinations). Similar to the giBCS, the iBCS will be based on key attributes describing the drug substance (solubility and permeability) and the drug product (dose and dissolution). This manuscript provides the foundational aspects of an iBCS, including the proposed scientific principles and framework upon which such a system can be developed.