Dual isotope SPECT-CT imaging using sodium-iodide symporter (NIS) reveals improved energetics following cardiac transplantation of stem cells encapsuled in hydrogels

Objectives Cell suspension prior to transplantation results in depression of cellular energetics and low engraftment.¹ Our aim is to design hydrogels that promote cardiosphere-derived cells (CDC) adhesion, restore metabolism, boost engraftment and to monitor in vivo transplanted CDC energetics with dual isotope SPECT-CT imaging.

Methods We designed bioadhesive hydrogels composed of 10% hyaluronic acid crosslinked by lysed blood/serum. In vitro studies: ¹⁸FDG uptake, ATP levels, oxygen consumption rate (OCR), extracellular acidification rate (ECAR) were measured in monolayers, hydrogels and 1hr suspended cells. In vivo energetics: Dual isotope SPECT-CT imaging (following intravenous ^99mTc and ²⁰¹Tl) was performed at 1hr & 24hrs following transplantation of NIS⁺CDCs.Reversible inhibition of AKT using MK2205 (10uM for 1hr) was performed to investigate hydrogel-mediated restoration of metabolism.

Results CDC dissociation/suspension induced energetic stress: we found reduction of ¹⁸FDG uptake (67%), ATP levels (48%), viability (81+3%) within 1hr. CDCs encapsulation in hydrogels restored energetics; we observed increase in ¹⁸FDG uptake (>2.5 fold), ATP levels (>6fold), ECAR/OCR (>1.5fold) within 1hr compared to suspended CDCs; MK2205 treatment results were similar to suspended CDCs In-vivo dual SPECT/CT imaging revealed that ^99mTc uptake was 60% lower at 1hr compared to 24hrs despite presence of lower numbers of cells at 24hrs. Encapsulation of CDCs in hydrogel resulted in high ^99mTc uptake at 1hr (1.6±0.4 kBq/mm³⁾, which was similar to the signal at 24hrs. MK2205 pretreatment resulted in low ^99mTc uptake at 1hr (1.03±0.32 kBq/mm³) and increase at 24hrs (1.65±0.37 kBq/mm³).

Conclusions In vivo dual isotope SPECT/CT imaging reveals impaired energetics of suspended CDCs. CDC encapsulation in hydrogels results in rapid restoration of metabolism and high viability following transplantation.