Molecular imaging of gene expression and protein function in vivo with PET and SPECT

Abstract

Molecular imaging is broadly defined as the characterization and measurement of biological processes in living animals, model systems, and humans at the cellular and molecular level using remote imaging detectors. One underlying premise of molecular imaging is that this emerging field is not defined by the imaging technologies that underpin acquisition of the final image per se, but rather is driven by the underlying biological questions. In practice, the choice of imaging modality and probe is usually reduced to choosing between high spatial resolution and high sensitivity to address a given biological system. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) inherently use image-enhancing agents (radiopharmaceuticals) that are synthesized at sufficiently high specific activity to enable use of tracer concentrations of the compound (picomolar to nanomolar) for detecting molecular signals while providing the desired levels of image contrast. The tracer technologies strategically provide high sensitivity for imaging small-capacity molecular systems in vivo (receptors, enzymes, transporters) at a cost of lower spatial resolution than other technologies. We review several significant PET and SPECT advances in imaging receptors (somatostatin receptor subtypes, neurotensin receptor subtypes, ␣ v ␤ 3 integrin), enzymes (hexokinase, thymidine kinase), transporters (MDR1 P-glycoprotein, sodium-iodide symporter), and permeation peptides (human immunodeficiency virus type 1 (HIV-1) Tat conjugates), as well as innovative reporter gene constructs (herpes simplex virus 1 thymidine kinase, somatostatin receptor subtype 2, cytosine deaminase) for imaging gene promoter activation and repression, signal transduction pathways, and protein-protein interactions in vivo.

Imaging reversal of multidrug resistance in living mice with bioluminescence: MDR1 P-glycoprotein transports coelenterazine

“…In effect, Pgp indirectly modulates intracellular Rluc enzyme kinetics by regulating the transmembrane translocation and availability of substrate. This fundamentally differs from readouts obtained with fluorescent probes such as rhodamine 123 and Hoechst 33342 (34,35), BCECF-AM and SPQ (36), or activated fluorescent compounds such as calcein-AM (37) or radiotracer substrates (23,38,39) that interact with Pgp. These probes and their output signals are retained for significant times after exposure of the cells to the probes, or alternatively, the Pgp-mediated readout depends on washout kinetics over time.…”

“…Noninvasive imaging of ATP-binding cassette transporters in living animals and humans has previously been reported with radiolabeled transport substrates and provides dynamic information on MDR1 Pgp and MRP1 functions in vivo (21,23). Selected radiopharmaceuticals have been validated in clinical studies for pretreatment assessment of Pgp-mediated MDR (32) and to directly visualize pharmacological modulation in patients (33).…”

“…In cancer, overexpression of MDR1 confers cross-resistance to a broad array of natural product cytotoxic agents (21). Pgp putatively acts as an energydependent efflux transporter that efficiently pumps drugs out of cells or off the plasma membrane, resulting in decreased intracellular accumulation of anticancer drugs and other substrates (22,23). In this study, we report that coelenterazine is a transport substrate recognized by human Pgp, thus enabling noninvasive bioluminescence imaging of Pgp transport activity in living animals.…”

Quantitative imaging of the T cell antitumor response by positron-emission tomography

“…Single-photon emission computed tomography (SPECT) detects ␥-emitting radioisotopes (28,29). PET detects positron emitters, has 10-to 100-fold more detection sensitivity than SPECT, and offers better spatial resolution (30).…”

The Integration of Positron Emission Tomography With Magnetic Resonance Imaging

“…PET reporter gene technologies rely on the binding or intracellular trapping of a radio-labeled moiety [94]- [96]. Substrates useful in other imaging modalites (SPECT, bioluminescence, and fluorescence) may be simultaneously incorporated to make a multimodal reporter gene construct [97], [98]. None of these imaging modalities offers the soft tissue contrast or physiological sensitivity of MRI.…”