Dose-Sparing Efficacy of d-Limonene with Low-Dose Allopurinol in a Dual Model of Hyperuricemia and Gouty Arthritis in Rats

<b>Background:</b><i>d</i>-Limonene (LIM) is a food-derived monoterpenoid phytocompound predominantly found in citrus peels, endowed with potent antioxidant and anti-inflammatory properties, and has been reported to inhibit xanthine oxidase (XO) activity in vitro. This study investigated the dose-sparing efficacy of this dietary bioactive compound in combination with low-dose allopurinol (ALP) using a dual rat model combining potassium oxonate (PO)-induced hyperuricemia and monosodium urate (MSU)-triggered gouty arthritis, thereby capturing both metabolic and inflammatory dimensions of gout. <b>Methods:</b> Female Wistar rats were PO-primed and MSU-challenged, then treated with LIM (50 mg/kg), ALP (5 or 10 mg/kg), or LIM + ALP. Outcomes included paw thickness, dysfunction and inflammation indices, serum uric acid, urea, creatinine, AST/ALT, cytokines (IL-1β, TNF-α, IL-6), oxidative stress markers (MDA, SOD, catalase, GSH), and <i>NLRP3</i> immunoreactivity, supported by radiographic and histopathological analyses. Data were analyzed by one-way ANOVA with Tukey's post hoc test. <b>Results:</b> LIM improved clinical and biochemical outcomes versus monotherapies. However, LIM + low-dose ALP exhibited the greatest overall efficacy. On Day 30, paw thickness was significantly lower with LIM + ALP than with LIM alone (3.25 ± 0.31 vs. 3.98 ± 0.72 mm; <i>p</i> < 0.001). Serum uric acid and hepatic transaminases declined most with the combination (<i>p</i> < 0.0001 vs. LIM), accompanied by improved renal indices (<i>p</i> < 0.001). Pro-inflammatory cytokines were markedly reduced, <i>NLRP3</i> immunostaining was minimal, and oxidative balance shifted toward homeostasis (↓ MDA; ↑ SOD, catalase, GSH). Radiographic and histological evaluations corroborated attenuation of joint inflammation and tissue damage. <b>Conclusions:</b> In the PO + MSU gout model, co-administration of the food-derived compound LIM with low-dose ALP achieved additive, dose-sparing benefits across metabolic, inflammatory, and histological endpoints. While in vivo XO activity was not directly assessed, the findings are consistent with <i>XO</i>-pathway modulation, NLRP3-IL-1β suppression, and redox restoration. These results highlight the potential of dietary bioactives such as <i>d</i>-Limonene to complement standard urate-lowering therapy, warranting further pharmacokinetic and safety validation.