Pharmacokinetic Considerations for Optimizing Inhaled Spray-Dried Pyrazinoic Acid Formulations

Abstract

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), remains a leading cause of death with 1.6 million deaths worldwide reported in 2021. Oral pyrazinamide (PZA) is an integral part of anti-TB regimens, but its prolonged use has the potential to drive the development of PZA-resistant Mtb. PZA is converted to the active moiety pyrazinoic acid (POA) by the Mtb pyrazinamidase encoded by pncA, and mutations in pncA are associated with the majority of PZA resistance. Conventional oral and parenteral therapies may result in subtherapeutic exposure in the lung; hence, direct pulmonary administration of POA may provide an approach to rescue PZA efficacy for treating pncA-mutant PZA-resistant Mtb. The objectives of the current study were to (i) develop novel dry powder POA formulations, (ii) assess their feasibility for pulmonary delivery using physicochemical characterization, (iii) evaluate their pharmacokinetics (PK) in the guinea pig model, and (iv) develop a mechanism-based pharmacokinetic model (MBM) using in vivo PK data to select a formulation providing adequate exposure in epithelial lining fluid (ELF) and lung tissue. We developed three POA formulations for pulmonary delivery and characterized their PK in plasma, ELF, and lung tissue following passive inhalation in guinea pigs. Additionally, the PK of POA following oral, intravenous, and intratracheal administration was characterized in guinea pigs. The MBM was used to simultaneously model PK data following administration of POA and its formulations via the different routes. The MBM described POA PK well in plasma, ELF, and lung tissue. Physicochemical analyses and MBM predictions suggested that POA maltodextrin was the best among the three formulations and an excellent candidate for further development as it has: (i) the highest ELF-to-plasma exposure ratio (203) and lung tissue-to-plasma exposure ratio (30.4) compared with POA maltodextrin and leucine (75.7/16.2) and POA leucine salt (64.2/19.3) and (ii) the highest concentration in ELF (C_maxELF: 171 nM) within 15.5 min, correlating with a fast transfer into ELF after pulmonary administration (K_PM: 22.6 1/h). The data from the guinea pig allowed scaling, using the MBM to a human dose of POA maltodextrin powder demonstrating the potential feasibility of an inhaled product.

Keywords: dry powder inhalation; pharmacokinetic; pyrazinamide; pyrazinoic acid; resistance; tuberculosis.

Figure 1.

Dosing scheme for pulmonary administration of POA formulations. POA dry powder for direct pulmonary delivery was administered every 3 min as indicated by the dark green arrows. Guinea pigs received a total of 8 doses administered every 3 min over 24 min. An air puff (10 mL) was administered every minute as indicated by the blue arrows, twice after each dose administration, to assist with aerosolization of the powder deposited in the chamber. Blood samples for quantification of POA in plasma were obtained after the last dose was administered (i.e., after 24 min).

Figure 2.

(A–E) Results of scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). SEM images showing the morphology of the spray-dried powder of (A) POA, (B) PM, (C) PML, and (D) PLS. (E) TGA thermograms for the different POA forms and excipients, pre-sprayed POA (black), pre-sprayed MAL (dark green), pre-sprayed LEU (aqua blue), SD POA (purple), PM (blue), PML (red), and PLS (green). MAL: maltodextrin, LEU: leucine, SD: spray-dried.

Figure 3.

Mechanism-based pharmacokinetic (PK) model to describe the disposition of pyrazinoic acid (POA) in plasma, lung tissue, and ELF following per oral (PO), intravenous (IV), intratracheal (IT), and pulmonary administration of POA and POA formulations. For IV and PO PK studies, POA was dosed as a solution. For the IT PK study, PM powder was dosed. For pulmonary administration, spray-dried dry powder POA formulations were used. CL_POA, POA clearance; V_POA, POA central volume of distribution; CL_d, POA intercompartmental clearance between lung and plasma; V_lung, volume of the lung tissue compartment; CL_ELF, POA intercompartmental clearance between lung tissue and ELF compartment; VELF, volume of ELF compartment; K_Form, first-order transfer rate from pulmonary depot to ELF following pulmonary administration of POA formulations; F_Form, bioavailability of POA formulations following pulmonary administration; K_IT, first-order transfer rate to ELF from IT depot following IT administration of PM; F_IT, bioavailability of intratracheally administered PM; Ka_oral, first-order absorption rate following oral administration of POA; F_Oral, bioavailability of orally administered POA; KP_Lung, lung to plasma partition coefficient.

Figure 4.

Mechanism-based model-predicted POA concentration versus time profiles following pulmonary administration of POA dry powder formulations (A) PM, (B) PML, and (C) PLS. Solid blue lines are model-predicted concentrations, while red dots are the observed POA concentrations. The figure shows the predicted concentration versus time profiles in plasma (top row), lung tissue (middle row), and ELF (bottom row). The dashed red line indicates the lower limit of quantification (LLOQ 202 nM). The shaded region represents the total duration of 24 min over which the POA formulation was dosed.