Oral solid dosage forms are often preferred during drug product development as of-fering a flexible and cost-effective solution for patients, hence improving patient ad-herence. The efficacy of an orally administered drug is nonetheless limited by its bioa-vailability. pH modification is an effective solution to improve solubility of weak acidic or basic drugs via the formation of a microenvironmental pH. The objective of this study was to evaluate different formulation approaches to develop an immediate-release tablet or capsule containing fumaric acid as pH-modifier for enhancing the bioavaila-bility of a weakly basic API. The goal was to maximize the bioavailability of the drug with acceptable amounts of pH-modifier to ensure manufacturability and reduced capsule / tablet size for improved patient experience. In vitro data suggested that the over-encapsulated tablet prototype led to improved bioavailability even in highly buff-ered achlorhydric conditions and was therefore the most robust formulation toward elevated gastric pH. However, in low buffered achlorhydric conditions the monolayer tablet with acid exhibited similar dissolution rate and prototypes with less close contact between the API and the organic acid also demonstrated improved dissolution rates. These results suggested that the acidic microenvironment may not require such a high degree of contact between the API and the pH-modifier. The outcomes of this study, and consequently the bio-relevance of tested media, will have to be further evaluated in vivo.