In Vivo Functional Assessment of Sodium-Glucose Cotransporters (SGLTs) Using [18F]Me4FDG PET in Rats

Abstract

Background: Mediating glucose absorption in the small intestine and renal clearance, sodium glucose cotransporters (SGLTs) have emerged as an attractive therapeutic target in diabetic patients. A substantial fraction of patients, however, only achieve inadequate glycemic control. Thus, we aimed to assess the potential of the SGLT-targeting PET radiotracer alpha-methyl-4-deoxy-4-[¹⁸F]fluoro-D-glucopyranoside ([¹⁸F]Me4FDG) as a noninvasive intestinal and renal biomarker of SGLT-mediated glucose transport.

Methods: We investigated healthy rats using a dedicated small animal PET system. Dynamic imaging was conducted after administration of the reference radiotracer 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG), or the SGLT-targeting agent, [¹⁸F]Me4FDG either directly into the digestive tract (for assessing intestinal absorption) or via the tail vein (for evaluating kidney excretion). To confirm the specificity of [¹⁸F]Me4FDG and responsiveness to treatment, a subset of animals was also pretreated with the SGLT inhibitor phlorizin. In this regard, an intraintestinal route of administration was used to assess tracer absorption in the digestive tract, while for renal assessment, phlorizin was injected intravenously (IV).

Results: Serving as reference, intestinal administration of [¹⁸F]FDG led to slow absorption with retention of 89.2 ± 3.5% of administered radioactivity at 15 min. [¹⁸F]Me4FDG, however, was rapidly absorbed into the blood and cleared from the intestine within 15 min, leading to markedly lower tracer retention of 18.5 ± 1.2% (P < 0.0001). Intraintestinal phlorizin led to marked increase of [¹⁸F]Me4FDG uptake (15 min, 99.9 ± 4.7%; P < 0.0001 vs. untreated controls), supporting the notion that this PET agent can measure adequate SGLT inhibition in the digestive tract. In the kidneys, radiotracer was also sensitive to SGLT inhibition. After IV injection, [¹⁸F]Me4FDG reabsorption in the renal cortex was significantly suppressed by phlorizin when compared to untreated animals (%ID/g at 60 min, 0.42 ± 0.10 vs. untreated controls, 1.20 ± 0.03; P < 0.0001).

Conclusion: As a noninvasive read-out of the concurrent SGLT expression in both the digestive tract and the renal cortex, [¹⁸F]Me4FDG PET may serve as a surrogate marker for treatment response to SGLT inhibition. As such, [¹⁸F]Me4FDG may enable improvement in glycemic control in diabetes by PET-based monitoring strategies.

Figure 1

Molecular structures of [¹⁸F]Me4FDG, [¹⁸F]FDG, and glucose and their interaction with SGLT.

Figure 2

Results of intestinal [¹⁸F]Me4FDG PET imaging. (a) Representative volume rendered image, coronal PET/CT fusion image, and coronal and transaxial volume of interest (VOI; light green area) of a rat. (b) Coronal maximum intensity projection images of dynamic [¹⁸F]Me4FDG PET over 15 min after radiotracer administration in untreated controls (upper rows) and after intraperitoneal (I.P., middle rows) and intra-intestinal (I.I., lower rows) treatment using the SGLT inhibitor phlorizin. In I.I. animals treated with this drug, tracer retention remained stable, while in untreated I.I.-administered controls, a markedly reduced radiotracer accumulation was observed over time. In the last column, [¹⁸F]FDG serving as a comparator also demonstrated slow radiotracer clearance in untreated animals. Quantitative comparison of radiotracer retention among all groups, displayed as (c) time-activity curves and (d) tracer retention at 15 min. Control [¹⁸F]Me4FDG group showed rapid decrease of I.I.-injected radioactivity (blue). I.I. phlorizin-pretreated [¹⁸F]Me4FDG group (red) showed little decrease, similar to (untreated) I.I.-injected [¹⁸F]FDG animals (green, serving as reference). I.P. phlorizin-pretreated [¹⁸F]Me4FDG rats (purple), however, demonstrated moderate decrease during image acquisition. Bars indicate mean ± SD values.

Figure 3

Results of renal [¹⁸F]Me4FDG PET imaging. (a) Multidirectional PET images of an untreated control rat versus a phlorizin- and canagliflozin-pretreated rat 10-20 min after intravenous radiotracer administration. For controls (upper rows), renal radiotracer accumulation in the cortex was substantially increased. In phlorizin-pretreated (middle rows) and canagliflozin-pretreated animals (lower rows), however, renal cortical and soft tissue activity was extremely low, while radioactivity in the renal pelvis and bladder was high. (b) Time-activity curves (cortex-to-pelvis count ratios) of all investigated rats, which revealed substantially lower renal cortical activity in pretreated animals over time. Comparison of the kidney (c) and blood radioactivity (d) at 60 min after the tracer administration among all three groups. In untreated controls, but not phlorizin- and canagliflozin-pretreated animals, the activity in the kidney and blood was substantially increased. Bars indicate mean ± SD values. %ID: percentage of injected dose.