**Background:** Ischemic cardiovascular disease is the leading cause of death worldwide. Current pharmacologic therapy has multiple limitations, and many patients remain symptomatic despite maximal medical therapy. Thymidine phosphorylase (TYMP), also known as platelet-derived endothelial cell growth factor, is an enzyme that catalyzes thymidine to thymine and 2-deoxy-D-ribose-1-phosphate. While TYMP has known proangiogenic activity, recent work showed that TYMP deficiency or inhibition reduces thrombosis in mice. This study aimed to comprehensively evaluate the effects of TYMP deficiency or inhibition on cardiac function, remodeling, and healing after acute myocardial infarction (AMI).
**Methods:** AMI was induced in male C57BL/6J wild-type (WT) and TYMP-deficient (Tymp-/-) mice (10-12 weeks old) by permanent ligation of the left anterior descending coronary artery (LAD). Cardiac function was assessed by echocardiography before surgery and at 7 and 28 days post-AMI. Langendorff isolated heart experiments evaluated contractility and coronary flow. Infarct size was measured by Evans blue and triphenyl tetrazolium chloride (TTC) staining at 18 hours and 4 weeks post-AMI. Microthrombus formation was assessed by CD41 immunofluorescence staining 24 hours post-AMI. Angiogenesis (CD31) and arteriogenesis (α-smooth muscle actin) were evaluated histologically. Bone marrow (BM) stem cell mobilization was assessed by colony formation assays. Mesenchymal stem cells (MSCs) were isolated from perigonadal fat pads and characterized for proliferation (cell counting, MTT assay), migration (wound healing assay), adhesion (static and flow chamber assays), and survival under hypoxia (CoCl2) and inflammation (TNF-α). Western blotting assessed GRIM-19 and AKT phosphorylation. In a separate experiment, WT mice were pretreated with tipiracil (1 mg/kg oral, daily) for 1 week before AMI and continued for 28 days post-AMI.
**Key Results:** Baseline cardiac function (ejection fraction, EF) was similar between WT and Tymp-/- mice (68.15% vs 65.88%, P=0.31). At 7 days post-AMI, EF declined similarly in both groups (57.96% vs 57.35%, P=0.81). However, at 28 days post-AMI, EF was significantly preserved in Tymp-/- mice compared to WT (62.85% vs 49.54%, P=0.001). Fractional shortening showed the same pattern (34.15% vs 24.99%, P=0.001). Systolic left ventricle anterior wall thickness (LVAWs) was significantly greater in Tymp-/- mice at both 7 days (1.27 mm vs 1.11 mm, P=0.03) and 28 days (1.53 mm vs 1.10 mm, P=0.001) post-AMI. Diastolic wall strain (DWS) was significantly higher in Tymp-/- mice at 28 days (0.33 vs 0.14, P=0.035), indicating reduced diastolic stiffness. Infarct size at 4 weeks was significantly reduced in Tymp-/- mice. Langendorff experiments showed TYMP-deficient hearts had slightly higher basal coronary flow (2.462 vs 1.984 mL/min) but lower developed pressure and dP/dt, suggesting reduced contractility may increase ischemic tolerance. Microthrombus formation at 24 hours post-AMI was significantly reduced in Tymp-/- hearts. Angiogenesis (CD31+ vessels) was unaffected, but arteriogenesis (α-SMA+ vessels) was increased in Tymp-/- hearts at 4 weeks. TYMP deficiency did not significantly alter expression of inflammatory cytokines at 3 days post-AMI. BM colony formation was dramatically enhanced in Tymp-/- mice 7 days post-AMI. Tymp-/- MSCs showed significantly increased proliferation, migration (wound closure faster by ~6 hours), and adhesion under flow conditions. Tymp-/- MSCs were resistant to high-dose CoCl2 (600 μM killed ~75% of WT MSCs but did not significantly affect Tymp-/- MSCs) and to TNF-α treatment. MMP2 activity was reduced in Tymp-/- MSC-conditioned media. Western blotting revealed reduced GRIM-19 expression and constitutively increased AKT phosphorylation in Tymp-/- MSCs. Tipiracil treatment phenocopied TYMP deficiency: EF at 4 weeks was 67.07% vs 49.54% in untreated WT (P=0.0005), with significantly improved LVAWs (1.37 vs 1.10 mm, P=0.034), DWS (0.30 vs 0.14, P=0.032), and reduced infarct size.
**Clinical Implications:** This study demonstrates that TYMP plays an adverse role after AMI and that its deficiency or inhibition preserves cardiac function through multiple mechanisms: reduced microthrombosis, enhanced BM stem cell mobilization, and generation of more resilient MSCs with improved proliferation, migration, adhesion, and survival under ischemic/inflammatory conditions. Tipiracil, an FDA-approved TYMP inhibitor, reproduced these benefits, suggesting it could be repurposed as a novel therapy for patients with AMI. The antiplatelet and antithrombotic effects of tipiracil are rapid and may be particularly valuable for patients requiring immediate antiplatelet intervention. These findings warrant further investigation in clinical settings.