Exploring the Enhanced Antiproliferative Activity of Turmeric Oil and 6-Mercaptopurine in a Combined Nano-Particulate System Formulation

6-Mercaptopurine (6-MP) is a chemotherapeutic agent with inadequate efficacy due to its poor aqueous solubility and limited bioavailability. Turmeric oil is a naturally occurring bioactive substance obtained from the rhizomes of Curcuma longa Linn that has well-known antiproliferative activities. The aim of this study was to develop a 6-MP-loaded turmeric oil-based self-nanoemulsifying drug delivery system (SNEDDS) to improve the anticancer activity of 6-MP. Turmeric oil was extracted and used in a range of 15–25% to develop SNEDDS formulations utilizing tween 80 and dimethyl sulfoxide as the surfactant and cosurfactant, respectively. The size, charge, and effect of the formulations on the viability against HepG2 and MCF-7 cell models, as well as the apoptosis and cell cycle, were analyzed. The prepared SNEDDS formulations were in the size range of 425.7 ± 7.4–303.6 ± 19.3 nm, using a polydispersity index of 0.429–0.692 and electronegative surface charges. Moreover, 6-MP-loaded SNEDDS with 15% turmeric oil content (F1) showed smaller particle sizes and a noticeable antiproliferative activity against both cell line models. Also, F1 showed a higher rate of late apoptosis than the pure drug and the corresponding non-medicated formulation. A morphological study revealed significant changes in the HepG2 cells compared to untreated cells. More cells halted in the S phase, and a marked decrease in the proportions of cells in the G1/G0 phase was observed when using SNEDDS formulation compared to pure drug. Thus, SNEDDS formulation is a promising drug delivery system for improving the antiproliferative activity of 6-MP, especially when turmeric oil is incorporated.