In vitro comparison of equine granulocytes labeled with 99mTc-hexamethylpropyleneamine oxime or 111In-oxine

Isolated equine granulocytes (WBC), radiolabeled with 99mTc-hexamethylpropyleneamine oxime (99mTc-HMPAO) or 111In-oxine, were evaluated in vitro for their labeling characteristics, viability, and phagocytic function over a 6-hour postlabeling period. Mean +/- SD labeling efficiency for 111In-oxine-WBC was 62.2 +/- 15.3%, which was significantly (P less than 0.001) higher than that for 99mTc-HMPAO-WBC (32.0 +/- 17.0%). In vitro elution of radiolabel from cells was significantly (P less than 0.02) greater for 99mTc-HMPAO-WBC at 0.5, 2, and 4 hours, but was not significantly different from elution of radiolabel for 111In-oxine-WBC at 6 hours. Viability, assessed by trypan blue dye exclusion, for 99mTc-HMPAO-WBC, 111In-oxine-WBC, and nonlabeled control WBC ranged from 97 to 100%, and was not significantly different among groups. Cell function was assessed by use of a phagocytosis assay and was reported as phagocytic index. The phagocytic index ranged from 0.86 to 0.96 for 99mTc-HMPAO-WBC, and from 0.76 to 0.97 for 111In-oxine-WBC. The phagocytic index was not significantly different at 0.5, 2, or 4 hours, but was significantly (P = 0.038) greater at 6 hours for 99mTc-HMPAO-WBC. Because of the superior imaging characteristics of 99mTc-HMPAO-WBC and equal or better labeling characteristics than those for 111In-oxine at 6 hours, 99mTc-HMPAO-WBC appear to be a good alternative to 111In-oxine-WBC.