Abstract 9919: PH-low Insertion Peptide (pHLIP) Targets Ischemic Myocardium

Abstract

pHLIP molecule (∼3 kD) inserts into a lipid bilayer when ambient pH is low. Originally, this property was used for targeted delivery of small molecules, liposomes and nanoparticles conjugated with pHLIP to visualize and treat tumors. Since cardiac ischemia causes a decrease in tissue pH we hypothesized that pHLIP can selectively bind to ischemic myocardium. METHODS. We studied pH-sensitive (Var7) and a pH-insensitive (kVar7) variants of pHLIP conjugated with fluorescent dye Alexa488 in three models. (1) Local myocardial ischemia was induced by repetitively occluding (10 min) and reperfusing (5 s) left coronary artery of anesthetized mice for 2 hr; 100 μL of 80 μM pHLIP were administered IV 3 min prior to the first occlusion. In the end, hearts were excised and retrogradely perfused with Evans Blue to delineate the area at risk. Hearts were frozen and sectioned to measure mean intensity of fluorescence (F). (2) Low flow (5% of normal flow rate) global ischemia was induced in Langendorff-perfused mouse hearts paced at 5 Hz and perfused with 1 μM pHLIP for 15 min. (3) Effects of 10 μM pHLIP on transmembrane potential and developed force were studied in isolated preparations of paced (4 Hz) murine left atrial appendages (AA) at normal and low (6.5) pH. RESULTS. In the model of local ischemia, when pH-sensitive Var7 was used, area of bright fluorescence coincided with the area at risk. F in the area at risk normalized to F in intact myocardium was higher with Var7 (3.89±.43, n=4) than with pH-insensitive kVar7 (1.11±.09, n=5, p.05). In global ischemia, F was higher in the hearts perfused with Var7 vs either kVar7 or Var7 in the absence of ischemia (Var7: 76±8, n=5; kVar7: 25±5, n=4; Var7 + no ischemia: 12±4, n=3, p<.05). In AA exposed to pH 6.5 (but not at pH 7.4), F was higher with Var7 than with kVar7 (76±9 vs 28±3, n=5; p<.05). Neither at the normal or low pH, pHLIP variants affected parameters of transmembrane potential or developed force. CONCLUSIONS. pH-sensitive pHLIP selectively binds to ischemic myocardium without affecting either ectrical or mechanical activity. Based on analogy with applications in oncology, pHLIP can be used for targeted delivery of contrasting agents and drugs to ischemic regions of the heart.