Discovery of IACS-52825, a Potent and Selective DLK Inhibitor for Treatment of Chemotherapy-Induced Peripheral Neuropathy.

Chemotherapy-induced peripheral neuropathy (CIPN) is a major unmet medical need with limited treatment options. Despite different mechanisms of action, diverse chemotherapeutics can cause CIPN through a converged pathway─an active axon degeneration program that engages the dual leucine zipper kinase (DLK). DLK is a neuronally enriched kinase upstream in the MAPK-JNK cascade, and while it is dormant under physiological conditions, DLK mediates a core mechanism for neuronal injury response under stress conditions, making it an attractive target for treatment of neuronal injury and neurodegenerative diseases. We have developed potent, selective, brain penetrant DLK inhibitors with excellent PK and activity in mouse models of CIPN. Lead compound IACS-52825 (22) showed strongly effective reversal of mechanical allodynia in a mouse model of CIPN and was advanced into preclinical development.

Inhibition of dual leucine zipper kinase prevents chemotherapy-induced peripheral neuropathy and cognitive impairments.

ABSTRACT: Chemotherapy-induced peripheral neuropathy (CIPN) and chemotherapy-induced cognitive impairments (CICI) are common, often severe neurotoxic side effects of cancer treatment that greatly reduce quality of life of cancer patients and survivors. Currently, there are no Food and Drug Administration-approved agents for the prevention or curative treatment of CIPN or CICI. The dual leucine zipper kinase (DLK) is a key mediator of axonal degeneration that is localized to axons and coordinates the neuronal response to injury. We developed a novel brain-penetrant DLK inhibitor, IACS'8287, which demonstrates potent and highly selective inhibition of DLK in vitro and in vivo. Coadministration of IACS'8287 with the platinum derivative cisplatin prevents mechanical allodynia, loss of intraepidermal nerve fibers in the hind paws, cognitive deficits, and impairments in brain connectivity in mice, all without interfering with the antitumor activity of cisplatin. The protective effects of IACS'8287 are associated with preservation of mitochondrial function in dorsal root ganglion neurons and in brain synaptosomes. In addition, RNA sequencing analysis of dorsal root ganglia reveals modulation of genes involved in neuronal activity and markers for immune cell infiltration by DLK inhibition. These data indicate that CIPN and CICI require DLK signaling in mice, and DLK inhibitors could become an attractive treatment in the clinic when coadministered with cisplatin, and potentially other chemotherapeutic agents, to prevent neurotoxicities as a result of cancer treatment.

Dual Leucine Zipper Kinase Is Constitutively Active in the Adult Mouse Brain and Has Both Stress-Induced and Homeostatic Functions.

Dual leucine zipper kinase (DLK, Map3k12) is an axonal protein that governs the balance between degeneration and regeneration through its downstream effectors c-jun N-terminal kinase (JNK) and phosphorylated c-jun (p-c-Jun). In peripheral nerves DLK is generally inactive until induced by injury, after which it transmits signals to the nucleus via retrograde transport. Here we report that in contrast to this mode of regulation, in the uninjured adult mouse cerebellum, DLK constitutively drives nuclear p-c-Jun in cerebellar granule neurons, whereas in the forebrain, DLK is similarly expressed and active, but nuclear p-c-Jun is undetectable. When neurodegeneration results from mutant human tau in the rTg4510 mouse model, p-c-Jun then accumulates in neuronal nuclei in a DLK-dependent manner, and the extent of p-c-Jun correlates with markers of synaptic loss and gliosis. This regional difference in DLK-dependent nuclear p-c-Jun accumulation could relate to differing levels of JNK scaffolding proteins, as the cerebellum preferentially expresses JNK-interacting protein-1 (JIP-1), whereas the forebrain contains more JIP-3 and plenty of SH3 (POSH). To characterize the functional differences between constitutive- versus injury-induced DLK signaling, RNA sequencing was performed after DLK inhibition in the cerebellum and in the non-transgenic and rTg4510 forebrain. In all contexts, DLK inhibition reduced a core set of transcripts that are associated with the JNK pathway. Non-transgenic forebrain showed almost no other transcriptional changes in response to DLK inhibition, whereas the rTg4510 forebrain and the cerebellum exhibited distinct differentially expressed gene signatures. In the cerebellum, but not the rTg4510 forebrain, pathway analysis indicated that DLK regulates insulin growth factor-1 (IGF1) signaling through the transcriptional induction of IGF1 binding protein-5 (IGFBP5), which was confirmed and found to be functionally relevant by measuring signaling through the IGF1 receptor. Together these data illuminate the complex multi-functional nature of DLK signaling in the central nervous system (CNS) and demonstrate its role in homeostasis as well as tau-mediated neurodegeneration.

Immunological characterization of HM5023507, an orally active PI3Kδ/γ inhibitor.

Phosphoinositide 3-kinases, delta (PI3Kδ) and gamma (PI3Kγ) are enriched in immune cells and regulate the development and function of innate and adaptive immunity. Dual PI3Kδγ inhibitors are considered high value targets for their potential to treat a variety of immune-mediated diseases, but their discovery has been challenging. Here we describe the preclinical pharmacology of HM5023507, an orally active dual inhibitor of δγ isoforms in immune signaling. HM5023507 inhibited PI3Kδ and PI3Kγ isoforms with greater than 100-fold selectivity against PI3Kα and PI3Kß in recombinant enzymatic assays and in primary human immune cells with an exquisite selectivity against other targets. HM5023507 attenuated the PI3Kδ/γ signaling in human basophils (IC50: 42/340 nmol/L; selectivity ratio ~1:8). HM5023507 attenuated the activation and function of human B and T cells, Th17 differentiation of CD4 T cells in the blood of healthy donors and rheumatoid arthritis patients, and cytokine and IgG production in human T and B cell cocultures, in vitro. Orally dosed HM5023507 attenuated PI3K δ/γ-mediated immune signaling in the rat in a dose-related manner. In addition, HM5023507 inhibited semiestablished collagen-induced arthritic inflammation in the rats (ED50 of 0.25mg/kg, p.o. BID or 0.5 mg/kg, QD, AUC: 1422 ng/mL*h), improved histopathology- and micro-computed tomography (µCT)-based indices of joint damage, bone destruction, and attenuated the levels of anti-collagen antibody, with an overall anti-inflammatory profile matching that of a TNFα neutralizing antibody. The PI3K δγ inhibitory profile of HM5023507 and its selectivity make it a useful tool to further delineate immunobiology of dual PI3K δγ targeting.

Functional and structural connectome properties in the 5XFAD transgenic mouse model of Alzheimer's disease.

Neurodegeneration in Alzheimer's disease (AD) is associated with amyloid-beta peptide accumulation into insoluble amyloid plaques. The five-familial AD (5XFAD) transgenic mouse model exhibits accelerated amyloid-beta deposition, neuronal dysfunction, and cognitive impairment. We aimed to determine whether connectome properties of these mice parallel those observed in patients with AD. We obtained diffusion tensor imaging and resting-state functional magnetic resonance imaging data for four transgenic and four nontransgenic male mice. We constructed both structural and functional connectomes and measured their topological properties by applying graph theoretical analysis. We compared connectome properties between groups using both binarized and weighted networks. Transgenic mice showed higher characteristic path length in weighted structural connectomes and functional connectomes at minimum density. Normalized clustering and modularity were lower in transgenic mice across the upper densities of the structural connectome. Transgenic mice also showed lower small-worldness index in higher structural connectome densities and in weighted structural networks. Hyper-correlation of structural and functional connectivity was observed in transgenic mice compared with nontransgenic controls. These preliminary findings suggest that 5XFAD mouse connectomes may provide useful models for investigating the molecular mechanisms of AD pathogenesis and testing the effectiveness of potential treatments.

Peripheral Administration of Tumor Necrosis Factor-Alpha Induces Neuroinflammation and Sickness but Not Depressive-Like Behavior in Mice.

Clinical observations indicate that activation of the TNF-α system may contribute to the development of inflammation-associated depression. Here, we tested the hypothesis that systemic upregulation of TNF-α induces neuroinflammation and behavioral changes relevant to depression. We report that a single intraperitoneal injection of TNF-α in mice increased serum and brain levels of the proinflammatory mediators TNF-α, IL-6, and MCP-1, in a dose- and time-dependent manner, but not IL-1ß. Protein levels of the anti-inflammatory cytokine IL-10 increased in serum but not in the brain. The transient release of immune molecules was followed by glial cell activation as indicated by increased astrocyte activation in bioluminescent Gfap-luc mice and elevated immunoreactivity against the microglial marker Iba1 in the dentate gyrus of TNF-α-challenged mice. Additionally, TNF-α-injected mice were evaluated in a panel of behavioral tests commonly used to study sickness and depressive-like behavior in rodents. Our behavioral data imply that systemic administration of TNF-α induces a strong sickness response characterized by reduced locomotor activity, decreased fluid intake, and body weight loss. Depressive-like behavior could not be separated from sickness at any of the time points studied. Together, these results demonstrate that peripheral TNF-α affects the central nervous system at a neuroimmune and behavioral level.

Effect of stress and peripheral immune activation on astrocyte activation in transgenic bioluminescent Gfap-luc mice.

Neuroinflammation and the accompanying activation of glial cells is an important feature of many neurodegenerative conditions. It is known that factors such as peripheral infections and stress can influence immune processes in the brain. However, the effect of these stressors on astrocyte activation in vivo remains elusive. In this study, transgenic Gfap-luc mice expressing the luciferase gene under the transcriptional control of the glial fibrillary acidic protein promoter were used to quantify the kinetics of in vivo astrocyte activation following immune challenges relevant to clinical inflammation. It was found that astrocytes respond rapidly to peripheral immune activation elicited by either bacterial lipopolysaccharide (LPS) or the viral mimetic polyinosinic:polycytidylic acid (poly(I:C)). By measuring bioluminescence and 18-kDa translocator protein radioligand binding in the same animal it was observed that LPS induces both astrocyte as well as microglial activation at 6 h post-administration. Furthermore, the astrocyte response decreased upon repeated systemic LPS injections, indicating development of tolerance to the LPS challenge. Finally, restraining Gfap-luc mice for 1 h daily on 5 consecutive days did not affect brain bioluminescence, thereby indicating that sub-chronic stress does not influence astrocyte activation under unchallenged conditions. However, stressed animals showed a reduced response to a subsequent systemic LPS injection, suggesting that the immune system is compromised in these animals. Here, we demonstrate that Gfap-luc mice can be used to study astrocyte activation in response to stimuli relevant for clinical inflammation and that this approach may provide a more complete characterization of existing and novel models of neuroinflammation

Systemic immune activation leads to neuroinflammation and sickness behavior in mice.

Substantial evidence indicates an association between clinical depression and altered immune function. Systemic administration of bacterial lipopolysaccharide (LPS) is commonly used to study inflammation-associated behavioral changes in rodents. In these experiments, we tested the hypothesis that peripheral immune activation leads to neuroinflammation and depressive-like behavior in mice. We report that systemic administration of LPS induced astrocyte activation in transgenic GFAP-luc mice and increased immunoreactivity against the microglial marker ionized calcium-binding adapter molecule 1 in the dentate gyrus of wild-type mice. Furthermore, LPS treatment caused a strong but transient increase in cytokine levels in the serum and brain. In addition to studying LPS-induced neuroinflammation, we tested whether sickness could be separated from depressive-like behavior by evaluating LPS-treated mice in a panel of behavioral paradigms. Our behavioral data indicate that systemic LPS administration caused sickness and mild depressive-like behavior. However, due to the overlapping time course and mild effects on depression-related behavior per se, it was not possible to separate sickness from depressive-like behavior in the present rodent model.

Itraconazole decreases left ventricular contractility in isolated rabbit heart: mechanism of action.

Itraconazole (ITZ) is an approved antifungal agent that carries a "black box warning" in its label regarding a risk of negative cardiac inotropy based on clinical findings. Since the mechanism of the negative inotropic effect is unknown, we performed a variety of preclinical and mechanistic studies to explore the pharmacological profile of ITZ and understand the negative inotropic mechanism. ITZ was evaluated in: (1) an isolated rabbit heart (IRH) preparation using Langendorff retrograde perfusion; (2) ion channel studies; (3) a rat heart mitochondrial function profiling screen; (4) a mitochondrial membrane potential (MMP) assay; (5) in vitro pharmacology profiling assays (148 receptors, ion channels, transporters, and enzymes); and (6) a kinase selectivity panel (451 kinases). In the IRH, ITZ decreased cardiac contractility (>30%) at 0.3µM, with increasing effect at higher concentrations, which indicated a direct negative inotropic effect upon the heart. It also decreased heart rate and coronary flow (≥1µM) and prolonged PR/QRS intervals (3µM). In mechanistic studies, ITZ inhibited the cardiac NaV channel (IC50: 4.2µM) and was devoid of any functional inhibitory effect at the remaining pharmacological targets. Lastly, ITZ did not affect MMP, nor interfere with mitochondrial enzymes or processes involved with fuel substrate utilization or energy formation. Overall, the cardiovascular and mechanistic data suggest that ITZ-induced negative inotropy is a direct effect on the heart, in addition, the potential involvement of mitochondria function and L-type Ca(2+) channels are eliminated. The exact mechanism underlying the negative inotropy is uncertain, and requires further study.

Design and synthesis of potent, orally efficacious hydroxyethylamine derived ß-site amyloid precursor protein cleaving enzyme (BACE1) inhibitors.

We have previously shown that hydroxyethylamines can be potent inhibitors of the BACE1 enzyme and that the generation of BACE1 inhibitors with CYP 3A4 inhibitory activities in this scaffold affords compounds (e.g., 1) with sufficient bioavailability and pharmacokinetic profiles to reduce central amyloid-ß peptide (Aß) levels in wild-type rats following oral dosing. In this article, we describe further modifications of the P1-phenyl ring of the hydroxyethylamine series to afford potent, dual BACE1/CYP 3A4 inhibitors which demonstrate improved penetration into the CNS. Several of these compounds caused robust reduction of Aß levels in rat CSF and brain following oral dosing, and compound 37 exhibited an improved cardiovascular safety profile relative to 1.

Ex vivo delta opioid receptor autoradiography: CNS receptor occupancy of two novel compounds over their antihyperalgesic dose range.

Discovered as part of an effort to identify delta opioid (DOPr or DOR) agonist analgesics, JNJ-20788560 and JNJ-39204880 exhibited high DOR affinity, with K(i) values of 1.7 and 2.0nM, respectively, and were selective for DOR over the mu opioid receptor (MOPr or MOR), with 596- and 122-fold selectivity, respectively. Both compounds stimulated DOR but not MOR induced GTPgammaS binding and were effective antihyperalgesic agents in the complete Freund's adjuvant model of thermal hyperalgesia in the rat, with oral ED(50) values of 13.5 and 35mg/kg, corresponding to plasma levels of 1 and 9microM, respectively. Autoradiographic analysis of DOR and MOR occupancy in sections of brain (striatum) and lumbar spinal cord (L4-L6) was determined ex vivo, using radiolabeled naltrindole or DAMGO. Quantitative image analysis resulted in striatal DOR ED(50) values of 6.9 and 10.7mg/kg, for JNJ-20788560 and JNJ-39204880 respectively, and spinal cord values of 6.4 and 3.2mg/kg, respectively. Neither compound dose-dependently occupied MOR within the dose range studied. Thus, this study confirmed the DOR selectively over MOR of both compounds following their oral administration, and further demonstrated dose-dependent DOR occupancy by each compound across its antihyperalgesic dose range. Importantly, these in vitro, in vivo, and ex vivo data revealed that the greater in vitro potency of JNJ-20788560 was paralleled by its greater in vivo potency, although JNJ-39204880 achieved higher plasma levels following its oral administration. The receptor occupancy levels observed at the pharmacologic ED(50) doses of these compounds suggest the need for greater target engagement by JNJ-39204880 than by JNJ-20788560 to elicit a similar therapeutic response.

Small-animal SPECT and SPECT/CT: important tools for preclinical investigation.

The need to study dynamic biologic processes in intact small-animal models of disease has stimulated the development of high-resolution nuclear imaging methods. These methods are capable of clarifying molecular interactions important in the onset and progression of disease, assessing the biologic relevance of drug candidates and potential imaging agents, and monitoring therapeutic effectiveness of pharmaceuticals serially within a single-model system. Single-photon-emitting radionuclides have many advantages in these applications, and SPECT can provide 3-dimensional spatial distributions of gamma- (and x-) ray-emitting radionuclide imaging agents or therapeutics. Furthermore, combining SPECT with CT in a SPECT/CT system can assist in defining the anatomic context of biochemical processes and improve the quantitative accuracy of the SPECT data. Over the past decade, dedicated small-animal SPECT and SPECT/CT systems have been developed in academia and industry. Although significant progress in this arena has been realized through system development and biologic application, further innovation continues to address challenges in camera sensitivity, spatial resolution, and image reconstruction and quantification. The innumerable applications of small-animal SPECT and SPECT/CT in drug development, cardiology, neurology, and oncology are stimulating further investment in education, research, and development of these dedicated small-animal imaging modalities.

Noninvasive assessment of myocardial viability in a small animal model: comparison of MRI, SPECT, and PET.

Acute myocardial infarction (AMI) research relies increasingly on small animal models and noninvasive imaging methods such as MRI, single-photon emission computed tomography (SPECT), and positron emission tomography (PET). However, a direct comparison among these techniques for characterization of perfusion, viability, and infarct size is lacking. Rats were studied within 18-24 hr post AMI by MRI (4.7 T) and subsequently (40-48 hr post AMI) by SPECT ((99)Tc-MIBI) and micro-PET ((18)FDG). A necrosis-specific MRI contrast agent was used to detect AMI, and a fast low angle shot (FLASH) sequence was used to acquire late enhancement and functional images contemporaneously. Infarcted regions showed late enhancement, whereas corresponding radionuclide images had reduced tracer uptake. MRI most accurately depicted AMI, showing the closest correlation and agreement with triphenyl tetrazolium chloride (TTC), followed by SPECT and PET. In some animals a mismatch of reduced uptake in normal myocardium and relatively increased (18)FDG uptake in the infarct border zone precluded conventional quantitative analysis. We performed the first quantitative comparison of MRI, PET, and SPECT for reperfused AMI imaging in a small animal model. MRI was superior to the other modalities, due to its greater spatial resolution and ability to detect necrotic myocardium directly. The observed (18)FDG mismatch likely represents variable metabolic conditions between stunned myocardium in the infarct border zone and normal myocardium and supports the use of a standardized glucose load or glucose clamp technique for PET imaging of reperfused AMI in small animals.

Alzheimer's disease-type neuronal tau hyperphosphorylation induced by A beta oligomers.

Alzheimer's disease (AD) is characterized by presence of extracellular fibrillar A beta in amyloid plaques, intraneuronal neurofibrillary tangles consisting of aggregated hyperphosphorylated tau and elevated brain levels of soluble A beta oligomers (ADDLs). A major question is how these disparate facets of AD pathology are mechanistically related. Here we show that, independent of the presence of fibrils, ADDLs stimulate tau phosphorylation in mature cultures of hippocampal neurons and in neuroblastoma cells at epitopes characteristically hyperphosphorylated in AD. A monoclonal antibody that targets ADDLs blocked their attachment to synaptic binding sites and prevented tau hyperphosphorylation. Tau phosphorylation was blocked by the Src family tyrosine kinase inhibitor, 4-amino-5-(4-chlorophenyl)-7(t-butyl)pyrazol(3,4-D)pyramide (PP1), and by the phosphatidylinositol-3-kinase inhibitor LY294002. Significantly, tau hyperphosphorylation was also induced by a soluble aqueous extract containing A beta oligomers from AD brains, but not by an extract from non-AD brains. A beta oligomers have been increasingly implicated as the main neurotoxins in AD, and the current results provide a unifying mechanism in which oligomer activity is directly linked to tau hyperphosphorylation in AD pathology.

Novel quantitative techniques for assessing regional and global function and structure based on modern imaging modalities: implications for normal variation, aging and diseased states.

In this review, we describe the current approaches used for quantitative assessment of regional and global function with positron emission tomography (PET) imaging (combined with structural imaging modalities) with emphasis on both research and clinical applications of this powerful approach. We particularly refer to the impact of such measurements in assessing physiological processes such as aging and measuring response to treatment in serious disorders such as cancer. Although a multitude of methods has been described in literature, the optimal approaches that are both accurate and practical in clinical settings need to be defined and refined. Standardized uptake value (SUV) continues to be the most widely used index in the current practice. Calculating SUV at a single time point and assigning standard regions of interest are inadequate and suboptimal for the purposes adopted by the medical community. The concepts of partial volume correction for measured values in small lesions, dual-time point and delayed PET imaging, and global metabolic activity for assessment of various stages of disease may overcome deficiencies that are associated with the current quantitative (ie, SUV) techniques. Serious consideration of these concepts will enhance the role and reliability of these quantitative techniques, and therefore compliment the World Health Organization or the Response Evaluation Criteria in Solid Tumors (RECIST) criteria for managing patients with cancer and other disorders, including physiological states such as aging and serious diseases such as atherosclerosis and neurological diseases. We also introduce the concepts that allow for segmentation of various structural components of organs like the brain for accurate measurement of functional parameters. We also describe complicated kinetic modeling and methodologies that have been used frequently for assessing metabolic and pharmacological parameters in the brain and other organs. Simplified quantitative techniques based on these concepts are described, but should be validated against the kinetic models to test their role as practical tools.

Coregistration of magnetic resonance and single photon emission computed tomography images for noninvasive localization of stem cells grafted in the infarcted rat myocardium.

This paper demonstrates the application of mutual information based coregistration of radionuclide and magnetic resonance imaging (MRI) in an effort to use multimodality imaging for noninvasive localization of stem cells grafted in the infarcted myocardium in rats. Radionuclide imaging such as single photon emission computed tomography (SPECT) or positron emission tomography (PET) inherently has high sensitivity and is suitable for tracking of labeled stem cells, while high-resolution MRI is able to provide detailed anatomical and functional information of myocardium. Thus, coregistration of PET or SPECT images with MRI will map the location and distribution of stem cells on detailed myocardium structures. To validate this coregistration method, SPECT data were simulated by using a Monte Carlo-based projector that modeled the pinhole-imaging physics assuming nonzero diameter and photon penetration at the edge. Translational and rotational errors of the coregistration were examined with respect to various SPECT activities, and they are on average about 0.50 mm and 0.82 degrees , respectively. Only the rotational error is dependent on activity of SPECT data. Stem cells were labeled with (111)Indium oxyquinoline and grafted in the ischemic myocardium of a rat model. Dual-tracer small-animal SPECT images were acquired, which allowed simultaneous detection of (111)In-labeled stem cells and of [(99m)Tc]sestamibi to assess myocardial perfusion deficit. The same animals were subjected to cardiac MRI. A mutual-information-based coregistration method was then applied to the SPECT and MRIs. By coregistration, the (111)In signal from labeled cells was mapped into the akinetic region identified on cine MRIs; the regional perfusion deficit on the SPECT images also coincided with the akinetic region on the MR image.

Solution state characterization of amyloid beta-derived diffusible ligands.

A growing body of evidence suggests that soluble oligomeric forms of the amyloid beta peptide known as amyloid-derived diffusible ligands (ADDLs) are the toxic species responsible for neurodegeneration associated with Alzheimer's disease. Accurate biophysical characterization of ADDL preparations is hampered by the peptide's strong tendency to self-associate and the effect of factors such as ionic strength, temperature, and pH on its behavior. In addition, amyloid peptides are known to interact with common laboratory excipients, specifically detergents, further complicating the results from standard analytical methods such as denaturing polyacrylamide gel electrophoresis. We have studied the solution behavior of various amyloid peptide preparations using analytical ultracentrifugation and size exclusion chromatography coupled with multiangle laser light scattering. Our results indicate that ADDL preparations exist in solution primarily as a binary mixture of a monomeric peptide and high-molecular mass oligomers. We relate our findings to previously described characterizations utilizing atomic force microscopy and electrophoretic methods and demonstrate that low-molecular mass oligomers identified by gel electrophoresis likely represent artifacts induced by the peptide's interaction with detergent, while atomic force microscopy results are likely skewed by differential binding of monomeric and oligomeric peptide species. Finally, we confirm that only the high-molecular mass oligomeric components of an ADDL preparation are capable of binding to subpopulations of primary hippocampal neurons in vitro.

Imaging stem cells implanted in infarcted myocardium.

Stem cell-based cellular cardiomyoplasty represents a promising therapy for myocardial infarction. Noninvasive imaging techniques would allow the evaluation of survival, migration, and differentiation status of implanted stem cells in the same subject over time. This review describes methods for cell visualization using several corresponding noninvasive imaging modalities, including magnetic resonance imaging, positron emission tomography, single-photon emission computed tomography, and bioluminescent imaging. Reporter-based cell visualization is compared with direct cell labeling for short- and long-term cell tracking.

Analytical derivation of the point spread function for pinhole collimators.

The point spread function (PSF) of a pinhole collimator plays an important role in determining the resolution and characterizing the sensitivity of the accepted photons from a given point in the image space. The focus of this paper is to derive an analytical expression for the PSF of two different types of focusing pinhole collimators that are based on (1) right-circular double cones and (2) oblique-circular double cones. Conventionally, focusing pinhole collimators used in multi-pinhole SPECT were designed using right-circular double cones, as they were easier to fabricate. In this work, a novel focusing collimator consisting of oblique-circular double cones was designed and its properties were studied in detail with respect to right-circular double-cone based collimators. The main advantage of determining the PSF is the fact that they can be used to accurately model the PSF during the reconstruction, thereby improving the resolution of the reconstructed image. The PSF of the focusing collimators based on oblique-circular cones were found to be almost shift invariant for low and medium energy photons (below 200 keV). This property is very advantageous as algorithms such as slice-by-slice reconstruction can be used for resolution recovery thereby drastically reducing the reconstruction time. However, the PSF of focusing oblique-circular double cones (FOCDC) for higher energy photons were found to be asymmetric and hence need to be modelled more accurately during the reconstruction. On the other hand, the PSF for the right-circular cone based collimators were found to be asymmetric for all energy levels. However, due to the smaller acceptance angle used, the number of penetration photons was found to be far less than that observed for oblique-circular cones. This results in a smaller PSF making right-circular cone based collimators preferable for high-resolution small animal imaging, especially where very small pinhole diameters are used. The analytically derived PSF for both collimators were validated using a ray-tracing based Monte Carlo approach and found to agree well with a mean square error of less than 1%.