Hypoxia in solid tumors is thought to be an important factor in resistance to therapy, but the extreme microscopic heterogeneity of the partial pressures of oxygen (pO₂) between the capillaries makes it difficult to characterize the scope of this phenomenon without invasive sampling of oxygen distributions throughout the tissue. Here we develop a non-invasive method to track spatial oxygen distributions in tumors during fractionated radiotherapy, using oxygen-dependent quenching of phosphorescence, oxygen probe Oxyphor PtG4 and the radiotherapy-induced Cherenkov light to excite and image the phosphorescence lifetimes within the tissue. Mice bearing MDA-MB-231 breast cancer and FaDu head neck cancer xenografts show different pO₂ responses during each of the 5 fractions (5 Gy per fraction), delivered from a clinical linear accelerator. This study demonstrates subsurface in vivo mapping of tumor pO₂ distributions with submillimeter spatial resolution, thus providing a methodology to track response of tumors to fractionated radiotherapy.

The Neuron Doctrine formulated by Ramón y Cajal at the turn of the 19th century is an embodiment of neuroscience discovery empowered by optics. Cajal used Camillo Golgi’s “black reaction” to sparsely impregnate neurons with light-absorbing silver nitrate in histological brain sections. Inspecting these sections with a light microscope, Cajal concluded that information was flowing in one specific direction—from dendrites to axons—within a network of discrete nerve cells connected by synapses. This conceptual advance, enabled through utilization of a light-absorbing probe in combination with widefield microscopy, laid the foundation for modern neuroscience. Fast-forward to the present day, and optical tools for measurement and manipulation of neuronal activity are so ubiquitous and versatile that “neurophotonics” has become a common word in the scientific vocabulary: we talk about neurophotonic technologies and research centers, we have the Neurophotonics journal, and recognize neurophotonics as its own field of research.

Molecules with strong absorption in the near-infrared (NIR) spectral region are of interest for a variety of applications in the photosciences. Nature's NIR absorbers are bacteriochlorophylls, which contain a tetrahydroporphyrin chromophore accentuated by attached auxochromes, including a β,meso-bridging isocyclic ring. Modification of the isocyclic ring has provided a convenient means of wavelength tuning. In other tetrapyrroles, annulation at the β,meso-, β,β-, and β-meso-β-positions also has enabled wavelength tuning. Here, synthetic bacteriochlorins bearing a gem-dimethyl group in each pyrroline ring have provided the foundation for studies of annulation. Seven new free base bacteriochlorins have been prepared and seven others have been elaborated therefrom. Six distinct routes known among porphyrins were explored. Ultimately, annulation of phenaleno or benzo groups at the β,meso-sites (i.e., spanning the 3,5- and 13,15-positions) of bacteriochlorins was achieved by a new route that entails two successive Pd-mediated coupling reactions of a dibromobacteriochlorin with a bromoarylboronic acid. The corresponding Phen-BC or Benz-BC absorbs at 913 or 1033 nm, respectively. Examination by time-resolved spectroscopy revealed an excited-state lifetime of 150 ps or 7 ps, with commensurably low fluorescence quantum yield (Φ_f) values estimated to be ∼0.004 or 3 × 10⁻⁵, respectively. The origin of the short excited-state lifetimes due to β,meso-annulation is unknown, but is not simply a consequence of the energy-gap law for non-radiative decay.

SignificanceCerebral metabolic rate of oxygen (CMRO2) consumption is a key physiological variable that characterizes brain metabolism in a steady state and during functional activation.AimWe aim to develop a minimally invasive optical technique for real-time measurement of CMRO2 concurrently with cerebral blood flow (CBF).ApproachWe used a pair of macromolecular phosphorescent probes with nonoverlapping optical spectra, which were localized in the intra- and extravascular compartments of the brain tissue, thus providing a readout of oxygen gradients between these two compartments. In parallel, we measured CBF using laser speckle contrast imaging.ResultsThe method enables computation and tracking of CMRO2 during functional activation with high temporal resolution (∼7 Hz). In contrast to other approaches, our assessment of CMRO2 does not require measurements of CBF or hemoglobin oxygen saturation.ConclusionsThe independent records of intravascular and extravascular partial pressures of oxygen, CBF, and CMRO2 provide information about the physiological events that accompany neuronal activation, creating opportunities for dynamic quantification of brain metabolism.

Dihydro analogues of BODIPYs exhibit spectral features (Φ_f ∼ 0.4–0.9) resembling aminocoumarins and suggest applications for broad-band photosensitization or where large Stokes shifts are desired.

The effects of tumor oxygenation on the outcome of radiotherapy have been under investigation for decades.^1,2 A multitude of studies have suggested that oxygen levels in tumors influence radiosensitivity of cells. Cancer cells in low-oxygen (hypoxic) environments can tolerate radiation doses 2 to 3 times higher than cells under normoxia.^3,4 Oxygen enhances the radiobiologic damage through the action of oxygen-derived radical species, while hypoxia induces signaling cascades leading to adaptation and resistance.^{5 – 7} Oxygenation of tumors can serve as a prognostic factor for survival after radiotherapy, as patients with hypoxic tumors commonly exhibit poorer outcomes.^8,9 Attempts have been made in numerous clinical trials to improve treatment outcomes by modifying oxygenation and/or the radiation dose delivered to tumors.^{10 – 12} To do that in an informed way, it is necessary to accurately quantify tumor oxygen levels, ideally periodically throughout the fractionation schedule. In this study, an approach to measuring oxygen during radiation delivery using small implantable sensors is examined that could be more easily deployed in clinics to aid radiotherapy verification and planning.