**1. Introduction**

Tuberculosis (TB) remains a major global health problem present in every country in the world, regardless of the availability of standard treatment guidelines [1,2]. It is the leading cause of death from a single infectious agent, ranking above the Human Immunodeficiency Virus (HIV) with about 10 million new active infections and 1.5–2 million fatalities annually [1,3–5]. TB is an airborne, highly contagious disease often spread by coughing and sneezing. It is caused by strains of bacteria known as *Mycobacterium tuberculosis* (Mtb), which primarily infects the lungs (pulmonary TB) and, occasionally, other body parts (extra-pulmonary TB) [1,5–7]. TB has been identified as a key cause of economic devastation, revolving poverty and illness that has entrapped families, societies, and even entire countries, with women, children, and HIV patients being the most vulnerable [2].

Tuberculosis is a major cause of childhood death. The World Health Organization (WHO) recently estimated that 10%–11% of the global population infected with TB are children, with about 233,000 childhood deaths each year [1,3,6]. The TB mortality rate is 70% higher in children under the age of five

than it is in adults in high burden areas [1,8]. Research indicates that children serve as reservoirs for active TB infection later in life, evidenced by the fact that globally, about 67 million children under the age of 15 have latent TB [1,9]. Latent TB infection is known as a state of persistent immune responses to stimulation by Mtb antigens with no evidence of clinical manifestations associated with active infection or symptoms of illness. Nevertheless, latent TB can develop into full-blown, active infection later [1].

Thus far, the greatest challenge to the successful treatment of TB in children is the significant shortage of e fficient pediatric pharmaceutical formulations [1,10–12]. Despite the alarming statistics reported on the number of active TB cases in children, it is indeed shocking to note that to date, childhood TB has been generally neglected worldwide, evidenced by treatment and clinical care schemes mostly extrapolated from studies in adults [1,13–15]. Over the years, children have been largely excluded from clinical trials resulting in weak evidence-based treatment of pediatric TB infection. With the shortage of suitable child-friendly anti-TB pharmaceutical formulations, it is common global practice to split adult fixed-dose combination (FDC) preparations: (i) into fractions; (ii) crushed to be taken with food, milk, and other liquids; or (iii) extemporaneous compounding to allow for easy use as needed per child. These practices can lead to dose inaccuracies, reduced active drug potencies, impaired dosage stability, irregular bioavailability, and poor compliance [14–16]. Consequently, there is an urgen<sup>t</sup> need for innovative treatment strategies that can contribute towards combating the TB epidemic in pediatric patients.

Isoniazid is the most widely used first-line anti-tubercular agen<sup>t</sup> for the treatment and prevention of TB infection in both children and adults. It is a drug of choice as it is bactericidal, easily administered, inexpensive, and relatively non-toxic in children. Isoniazid is almost completely absorbed from the gastrointestinal tract and penetrates all body fluid cavities, in which drug levels are similar to serum levels [12,15]. Although subject to considerable hepatic metabolism (or first-pass e ffect) after oral dosing, it reaches concentrations well above the minimum inhibitory levels of Mtb in most tissues and TB lesions when given in standard doses [17]. Isoniazid is considered a class III drug according to the Biopharmaceutics Classification System (BCS) [18], meaning that it is highly hydrophilic (aqueous solubility = 125 mg/mL at 25 ◦C) [18,19] but not very permeable (log P = −0.64 at 25 ◦C) [19]. It is weakly basic, crystalline in nature, and does not display polymorphism [20]. The aim of this study was, therefore, to develop and characterize a microparticulate reconstitutable dry suspension containing isoniazid as a model drug. To date, there is not much detailed in literature on the use of anti-tubercular micro-suspensions for pediatric TB treatment or prophylaxis. Conventional dry suspensions are powder mixtures that require the addition of water at the time of dispensing. They are often widely acceptable, intended for oral administration, and usually choice alternatives when drug stability is a major concern. Dry suspensions are easy to use by any age group (particularly children) and, therefore, enhance patient compliance [13,21]. In this study, microparticulate dry suspensions as micro-structuring remains an ideal way of manufacturing highly e fficient, rate-modulated pharmaceutical formulations that are beneficial to patients. Generally, microparticulate drug carriers are known to have high stability with excellent drug loading capacities for hydrophobic and hydrophilic drug moieties, enhanced bioavailability, and decreased toxicity [22]. The reconstitutable dry suspension (RDS) was prepared by blending liquid and solid interphases of drug and excipient into a homogenous mix that was lyophilized and pulverized to produce an isoniazid-loaded free-flowing powdery formulation. Formulation evaluation involved zeta potential and polydispersity index analyses, particle sizing, drug loading, dissolution testing, thermal behavior, structural transitions, surface morphology, crystallinity determinations, cytotoxicity, hydro- and environmental- stability testing.

### **2. Materials and Methods**
