Nitric oxide imaging in cancer enabled by steric relaxation of a photoacoustic probe platform†
Nitric oxide (NO) is a key signaling molecule involved in a variety of physiological and pathological processes. Over the past few decades it has become clear that the microenvironment and concentration of NO are critically linked to its bioactivity. Direct visualization of NO in vivo remains difficult due to a lack of sensitive analytical tools with deep tissue compatibility. Herein, we report the optimization of an activatable photoacoustic probe for NO by planarizing the boron-azadipyrromethene (aza-BODIPY) dye platform via steric relaxation. The lead compound, SR-APNO-3, exhibits maximal absorption at 790 and 704 nm before and after N-nitrosation with NO, respectively, and a 4.4-fold increase in the theoretical maximal ratiometric response compared to the non-sterically relaxed parent compound, APNO-5. This circa 30 nm red-shift enabled direct visualization with the laser system commonly employed in commercially available photoacoustic tomographers and a 1.1-fold increase in sensitivity within an intramuscular lipopolysaccharide-induced inflammation model. Moreover, this optimization facilitated the detection of endogenously produced NO in an allograft murine breast cancer model, where steady-state concentrations are several orders of magnitude less than during the immune response.