Activatable photoacoustic lifetime probe for imaging of enzyme activity

Abstract

Photoacoustic molecular imaging (PMI) is a non-ionizing imaging modality that can provide functional and metabolic information at clinically relevant penetration depth (~ 2 cm) with submillimeter spatial resolution. This doctoral dissertation focuses on applying PMI to measure the activity of matrix metalloproteinase 2 (MMP-2), an enzymatic marker of cancer progression capable of altering the tumor microenvironment. Photoacoustic lifetime imaging (PALI) is used to probe the transient absorption of an activatable probe capable of changing its excited-state lifetime from short (1 – 10 ns) to long (1 – 10 µs) upon cleavage by MMP-2. Several models were developed to test the hypothesis of lifetime contrast between the inactive and active form of the probe. A series of peptide-based activatable photoacoustic probes of varying length and structure were synthesized. In order to enlighten the structure/function relationship and identify the optimal probe, we compared their dimerization and separation efficiency, activation rate, and photoacoustic lifetime signal strength with respect to a non-activatable control. Our long-term goal is to implement this technique to image the activity of enzymes involved in pathological processes in clinical settings. To this end, we have developed a portable multimodal imaging system for the simultaneous visualization of ultrasound anatomical images and PALI enzymatic activity. Ultimately, this technique could be translated into an early-stage cancer diagnostic or prognostic tool, as well as for personalized treatment planning or continuous monitoring of treatment efficacy.