RT Journal Article SR Electronic T1 Sex as a Biologic Variable in Preclinical Imaging Research: Initial Observations with 18F-FLT JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 833 OP 838 DO 10.2967/jnumed.117.199406 VO 59 IS 5 A1 Szeman Ruby Chan A1 Kelley Salem A1 Justin Jeffery A1 Ginny L. Powers A1 Yongjun Yan A1 Kooresh I. Shoghi A1 Aparna M. Mahajan A1 Amy M. Fowler YR 2018 UL http://jnm.snmjournals.org/content/59/5/833.abstract AB The study objective was to investigate whether sex influences 3′-deoxy-3′-18F-fluorothymidine (18F-FLT) uptake and tissue distribution in mouse models of cancer. Methods: 18F-FLT biodistribution was measured in 3 strains of male and female mice (129S6/SvEv, athymic nude, and BALB/c). 18F-FDG biodistribution was measured for comparison. 18F-FLT uptake was also measured in female 129S6/SvEv mice bearing estrogen-dependent SSM3 mouse mammary tumors, male athymic nude mice bearing androgen-dependent CWR22 prostate cancer xenografts, and male and female athymic nude mice bearing estrogen-independent MDA-MB-231 human breast cancer xenografts. Ki-67 expression was assayed by immunohistochemistry. PET/CT imaging was performed to visualize 18F-FLT biodistribution and to determine pharmacokinetics. Results: Greater 18F-FLT activity was observed in blood, liver, muscle, heart, kidney, and bone in female than male mice. Pharmacokinetic analysis demonstrated higher early renal 18F-FLT activity and greater accumulation of 18F-FLT in the urinary bladder in male than female mice. The differential pattern of 18F-FLT biodistribution between the sexes seen with 18F-FLT was not observed with 18F-FDG. Increased tumoral 18F-FLT uptake compared with muscle was observed in both the SSM3 mammary tumors (2.4 ± 0.17 vs. 1.6 ± 0.14 percentage injected dose [%ID]/g at 2 h after injection, P = 0.006) and the CWR22 prostate cancer xenografts (0.34 ± 0.08 vs. 0.098 ± 0.033 %ID/g at 2 h after injection, P = 0.03). However, because of higher nonspecific muscle uptake in female mice, tumor-to-muscle uptake ratios were greater for CWR22 tumors than for SSM3 tumors (4.2 ± 0.78 vs. 1.5 ± 0.049 at 2 h after injection, P = 0.008). Sex-dependent differences in 18F-FLT uptake were also observed for MDA-MB-231 xenografts (tumor-to-muscle ratio, 7.2 ± 0.9 for female vs. 16.9 ± 8.6 for male, P = 0.039). Conversely, greater tumoral Ki-67 staining was observed in female mice (71% ± 3% for female vs. 54% ± 2% for male, P = 0.009), and this finding more closely matched the relative differences in absolute 18F-FLT tumor uptake values (4.5 ± 0.99 %ID/g for female vs. 1.9 ± 0.30 %ID/g for male, P = 0.03). Conclusion: Depending on whether female or male mice are used, differences in biodistribution and nonspecific tissue uptake can adversely affect quantitative measures of 18F-FLT uptake. Thus, sex is a potential variable to consider in defining quantitative imaging metrics using 18F-FLT to assess tumor proliferation.