Imaging colon cancer response following treatment with AZD1152: a preclinical analysis of [18F]fluoro-2-deoxyglucose and 3'-deoxy-3'-[18F]fluorothymidine imaging

Abstract

Purpose: To determine whether treatment response to the Aurora B kinase inhibitor, AZD1152, could be monitored early in the course of therapy by noninvasive [(18)F]-labeled fluoro-2-deoxyglucose, [(18)F]FDG, and/or 3'-deoxy-3'-[(18)F]fluorothymidine, [(18)F]FLT, PET imaging.

Experimental design: AZD1152-treated and control HCT116 and SW620 xenograft-bearing animals were monitored for tumor size and by [(18)F]FDG, and [(18)F]FLT PET imaging. Additional studies assessed the endogenous and exogenous contributions of thymidine synthesis in the two cell lines.

Results: Both xenografts showed a significant volume-reduction to AZD1152. In contrast, [(18)F]FDG uptake did not demonstrate a treatment response. [(18)F]FLT uptake decreased to less than 20% of control values in AZD1152-treated HCT116 xenografts, whereas [(18)F]FLT uptake was near background levels in both treated and untreated SW620 xenografts. The EC(50) for AZD1152-HQPA was approximately 10 nmol/L in both SW620 and HCT116 cells; in contrast, SW620 cells were much more sensitive to methotrexate (MTX) and 5-Fluorouracil (5FU) than HCT116 cells. Immunoblot analysis demonstrated marginally lower expression of thymidine kinase in SW620 compared with HCT116 cells. The aforementioned results suggest that SW620 xenografts have a higher dependency on the de novo pathway of thymidine utilization than HCT116 xenografts.

Conclusions: AZD1152 treatment showed antitumor efficacy in both colon cancer xenografts. Although [(18)F]FDG PET was inadequate in monitoring treatment response, [(18)F]FLT PET was very effective in monitoring response in HCT116 xenografts, but not in SW620 xenografts. These observations suggest that de novo thymidine synthesis could be a limitation and confounding factor for [(18)F]FLT PET imaging and quantification of tumor proliferation, and this may apply to some clinical studies as well.

Fig 1

Sequential imaging of representative HCT116 flank xenografts (dotted circle). [¹⁸F]FDG (A) and [¹⁸F]FLT (B) microPET 2D transaxial images through a HCT116 xenograft from the same control animal and from the same AZD1152-treated animal are shown. * The first [¹⁸F]FDG study in the treatment group (day 7) was performed under anesthesia during the entire study; in all other studies the animals were awake after radiotracer injection until time of imaging. Note the central area of necrosis (low tracer uptake) as the xenografts get larger.

Fig 2

Sequential imaging of representative SW620 flank xenografts (dotted circle). [¹⁸F]FDG uptake (A) and [¹⁸F]FLT (B) microPET 2D transaxial images through a SW620 xenograft from the same control animal and from the same AZD1152-treated animal are shown. Note the central area of necrosis (low tracer uptake) as the xenografts get larger.

Fig 3

Measurements of [¹⁸F]FDG and [¹⁸F]FLT uptake in HCT116 xenografts. Units are %ID/cc of the maximum-voxel value (A, C), and µCi of total tumor radioactivity (B, D). Hatched bar represents non-treated, control animals; solid bar represents AZD1152-treated animals. Growth profiles of HCT116 s.c. xenografts (E). Open circles represent the mean tumor volume of untreated (control) animals. Closed circles represent the mean tumor volume of AZD1152-treated animals. Arrows show the beginning of each two-day AZD1152 treatment cycle; the day post implantation is shown in the abcissa. * Indicates that the first [¹⁸F]FDG study in the treatment group (day 7) was performed under anesthesia during the entire study; in all other studies the animals were awake after radiotracer injection until time of imaging.

Fig 4

Measurements of [¹⁸F]FDG and [¹⁸F]FLT uptake (%dose/g) in SW620 xenografts. Units are %ID/cc of the maximum-voxel value (A, C), and µCi of total tumor radioactivity (B, D). normalized uptake ratios of xenograft-to-surrounding tissue (B, D). Hatched bar represents non-treated, control animals; solid bar represents AZD1152-treated animals. Growth profiles of SW620 s.c. xenografts (E). Open circles represent the mean tumor volume of untreated (control) animals. Closed circles represent the mean tumor volume of AZD1152-treated animals. Arrows shows the beginning of each two-day AZD1152 treatment cycle; the day post implantation is shown in the abcissa.

Fig 5

Immunohistochemistry. H&E and Ki67 staining of tumor samples acquired from experimental animals bearing HCT116 and SW620 xenografts at the different time points of the study.

Fig. 6

In vitro assessments. EC50 estimates for AZD1152-HQPA, methotrexate (MTX) and 5-fluorouracil (5FU) in HCT116 and SW620 cell lines (A). Immunoblots for thymidine kinase (TK), thymidylate synthase (TS) and α tubulin in HCT116 and SW620 cells (B). [³H]FLT, [¹⁴C]TdR and [^99mTc]DTPA accumulation in HCT116 and SW620 cells (C); data are plotted as radioactivity-time profiles for each tracer; tracer uptake is expressed as (µL medium/ mg total cell protein). Proliferation-time profiles for HCT116 and SW620 cells (D).