PK 11195 and
DAA1106 bind with high-affinity to the translocator
protein (TSPO, formerly known as the peripheral
benzodiazepine receptor). TSPO expression in glial cells increases in response to
cytokines and pathological stimuli. Accordingly, [(11)C]-
PK 11195 and [(11)C]-
DAA1106 are recognized molecular imaging (MI) agents capable of monitoring changes in TSPO expression occurring in vivo and in response to various neuropathologies.Here we tested the pharmacological characteristics and TSPO-monitoring potential of two novel MI agents:
NIR-conPK and
NIR-6T.
NIR-conPK is an analogue of
PK 11195 conjugated to the near-infrared (NIR) emitting fluorophore:
IRDye 800CW.
NIR-6T is a
DAA1106 analogue also conjugated to
IRDye 800CW.We found that
NIR-6T competed for [(3)H]-
PK 11195 binding in
astrocytoma cell homogenates with nanomolar affinity, but did not exhibit specific binding in intact
astrocytoma cells in culture, indicating that
NIR-6T is unlikely to constitute a useful MI agent for monitoring TSPO expression in intact cells. Conversely, we found that
NIR-conPK did not compete for [(3)H]-
PK 11195 binding in
astrocytoma cell homogenate, but exhibited specific binding in intact
astrocytoma cells in culture with nanomolar affinity, suggesting that
NIR-conPK binds to a
protein distinct, but related to, TSPO. Accordingly, treating intact
astrocytoma cells and microglia in culture with
cytokines led to significant changes in the amount of
NIR-conPK specific binding without corresponding change in TSPO expression. Remarkably, the
cytokine-induced changes in the
protein targeted by
NIR-conPK in intact microglia were selective, since IFN-gamma (but not
TNFalpha and
TGFbeta) increased the amount of
NIR-conPK specific binding in these cells.Together these results suggest that
NIR-conPK binds to a
protein that is related to TSPO, and expressed by
astrocytomas and microglia. Our results also suggest that the expression of this
protein is increased by specific
cytokines, and thus allows for the monitoring of a particular subtype of microglia activation.