Biodistribution, pharmacokinetics, and nuclear imaging studies of 111In-labeled rGel/BLyS fusion toxin in SCID mice bearing B cell lymphoma

Abstract

Purpose: We examined the biodistribution and pharmacokinetics of (111)In-labeled rGel/BLyS, a gelonin toxin (rGel)-B lymphocyte stimulator (BLyS) fusion protein.

Materials and methods: rGel/BLyS was labeled with In-111 through DTPA with a labeling efficiency >95%. Biodistribution/imaging studies were obtained in severe-combined immunodeficiency mice bearing diffuse large B cell lymphoma OCI-Ly10. Pharmacokinetic studies were performed in BALB/c mice.

Results: In vitro, DTPA-conjugated rGel/BLyS displayed selective cytotoxicity against OCI-Ly10 cells and mantle cell lymphoma JeKo cells. In vivo, rGel/BLyS exhibited a tri-exponential disposition with a rapid initial mean distribution followed by an extensive mean distribution and a long terminal elimination phase. At 48 h after injection, uptake of the radiotracer in tumors was 1.25 %ID/g, with a tumor-to-blood ratio of 13. Tumors were clearly visualized at 24-72 h post-injection. Micro-SPECT-CT images and ex vivo analyses confirmed the accumulation of rGel/BLyS in OCI-Ly10 tumors.

Conclusions: (111)In-DTPA-rGel/BLyS are distributed to B cell tumors and induce apoptosis in tumors. Preclinical antitumor studies using rGel/BLyS should use a twice-per-week treatment schedule.

Fig. 1.

Cytotoxicities of DTPA-rGel/BLyS, rGel/BLyS, and rGel after 96-h incubation in Jeko-1 and OCI-Ly10 cells. No significant difference in inhibition of cell growth was observed after DTPA modification of rGel/BLyS compared to the unmodified rGel/BLyS.

Fig. 2.

Radio-instant thin layer chromatography revealed that the radiochemical purity of ¹¹¹In-DTPA-rGel/BLyS was greater than 95% after purification with a PD-10 column. The specific activity of the radio-labeled conjugate was 0.3 MBq/μg (8.1 μCi/μg).

Fig. 3.

The mean blood concentration-time profile of ¹¹¹In-DTPA-rGel/BLyS generated using WinNonlin software. The data showed a tri-exponential disposition of the drug following intravenous administration.

Fig. 4.

Biodistribution of ¹¹¹In-DTPA-rGel/BLyS at 48 h after intravenous injection of the radiotracer in SCID mice bearing OCI-Ly10 tumors. Data are expressed as percentage of injected dose per gram of tissue (%ID/g), represented as mean ± standard deviation (n = 5).

Fig. 5.

(A) Representative μSPECT-CT images of a mouse bearing an OCI-Ly10 tumor after intravenous injection of ¹¹¹In-DTPA-rGel/BLyS. The radiotracer primarily accumulated in the spleen, liver, and kidney. Uptake in the tumor is clearly visualized at 24, 48, and 72 h post-injection. (B) Accumulation of ¹¹¹In-DTPA-rGel/BLyS in OCI-Ly10 tumor at different times after intravenous administration of ¹¹¹In-DTPA-rGel/BLyS at a dose of 48 μg per mouse. The percentage of injected radioactivity in each tumor was calculated on the basis of gamma images and quantitative analysis of the counts per pixel in the tumors. The tumor uptake of ¹¹¹In-DTPA-rGel/BLyS steadily increased over time and peaked at 24 h post-injection.

Fig. 5.

(A) Representative μSPECT-CT images of a mouse bearing an OCI-Ly10 tumor after intravenous injection of ¹¹¹In-DTPA-rGel/BLyS. The radiotracer primarily accumulated in the spleen, liver, and kidney. Uptake in the tumor is clearly visualized at 24, 48, and 72 h post-injection. (B) Accumulation of ¹¹¹In-DTPA-rGel/BLyS in OCI-Ly10 tumor at different times after intravenous administration of ¹¹¹In-DTPA-rGel/BLyS at a dose of 48 μg per mouse. The percentage of injected radioactivity in each tumor was calculated on the basis of gamma images and quantitative analysis of the counts per pixel in the tumors. The tumor uptake of ¹¹¹In-DTPA-rGel/BLyS steadily increased over time and peaked at 24 h post-injection.

Fig. 6.

Representative autoradiogram and histological analysis of an OCI-Ly10 tumor from a mouse that received intravenous administration of ¹¹¹In-DTPA-rGel/BLyS at 72 h before tumor removal. (A) Autoradiographic image depicts the intratumor distribution pattern of the radiolabeled rGel/BLyS, which was confirmed by H&E staining (B) of the same section. (C) Immunohistochemical staining of regions 1, 2, and 3 in (A) at higher magnification. The slides adjacent to the slide used for autoradiographic study were stained with TUNEL for apoptotic cells, BAFF-R antibody and anti-rGel antibody, respectively. Each marker was pseudo-colored green.