**1. Introduction**

A drug compound is classified using the biopharmaceutical categorization system according to its permeability and solubility in water [1]. When making decisions on the discovery and early development of new drugs, the Biopharmaceutical Classification System is a helpful resource. By categorizing drug compounds into four classes backed by their solubility associated with dose and intestinal permeability in combination with the dissolution properties of the dosage form, it enables

the prediction of *in vivo* pharmacokinetics of oral immediate-release (IR) medicinal products [2]. The US Food and Drug Administration (FDA) guidelines for the Biopharmaceutical Classification System (BCS) were made available to increase the effectiveness of the drug product development process [3]. The Biopharmaceutical Classification Technique is a system for categorizing medicines according to their permeability and solubility. The US Food and Drug Administration has supplied it as a forecasting tool for intestinal drug absorption.

Oral administration is the most suitable and commonly employed route of drug delivery thanks to its simple administration, patient acceptability, cost efficiency, least sterility maintenance, and flexibility in the design of a dosage form. However, the main disadvantage with the formulating of oral drug delivery system lies with their poor oral bioavailability. The oral bioavailability of drug substances lies on several parameters including water solubility, drug permeability across biological membrane, dissolution rate, pre-systemic metabolism [4].

It is a factor that the poor water solubility and dissolution profile of drugs in GI fluid often causes the poor bioavailability. The oral bioavailability of drugs may be improved by increasing the aqueous solubility and dissolution profile of the drug substance in the GI fluids. As far as considering the BCS Class II drugs, the rate-limiting step is the drug release from the formulation in GI fluid and not the absorption from biological membrane; therefore, increase in the solubility may increase the bioavailability of BCS Class II drugs [5–8].

Only 1% of medication compounds entered the market in the pharmaceutical sector, and this is always due to inadequate biopharmaceutical qualities rather than toxicity or a lack of therapeutic efficacy [9–12]. Solubility is one of these biopharmaceutical qualities that is a big problem, because of their weak solubility, medications are always useless during production that can be sold. Increasing the solubility of medication ingredients is currently one of the pharmaceutical company's biggest concerns. Particle size reduction is one of the techniques that have been utilized to increase the water solubility of pharmaceuticals [5, 6]. Creating salt [7], emulsification [8, 9], co-solvent solubilizations [10], and employing polymers to transport medications that are not highly water soluble [11] are some examples. Even though it has been demonstrated that these techniques increase oral bioavailability, their effectiveness depends on the particular physicochemical characteristics of the medications under investigation [13, 14]. Pharmaceutical co-crystal formulation has been increasing interest over the past few years as a potential means of enhancing the bioavailability of medications with poor water solubility. Co-crystal and pharmaceutical co-crystal are two terms that must first be understood. There are several ways to define co-crystals [15]. Co-crystals are defined as structurally homogeneous/heterogeneous crystalline solids that include drug and coformer in specific stoichiometric proportions. The discrete neutral molecular reactants that make up co-crystals are solids at room temperature. According to this definition of co-crystals, a pharmaceutical co-crystal is a mixture in which one of the co-crystals' elements serves as an active medicinal ingredient and the other elements serve as coformers. An active drug hydrate is not a co-crystal, as is evident from the statement, but a solid-state drug hydrate is co-crystalline with a coformer to produce a co-crystal. [16]. The pharmaceutical sector currently places a lot of attention on co-crystal methods. Pharmaceutical co-crystals can successfully enhance the drug substance's solubility, dissolving profile, bioavailability and physical stability, in addition to other crucial features such as flowability, chemical stability, compressibility and hygroscopicity [17].
