LXR/CD38 activation drives cholesterol-induced macrophage senescence and neurodegeneration via NAD+ depletion

Although dysregulated cholesterol metabolism predisposes aging tissues to inflammation and a plethora of diseases, the underlying molecular mechanism remains poorly defined. Here, we show that metabolic and genotoxic stresses, convergently acting through liver X nuclear receptor, upregulate CD38 to promote lysosomal cholesterol efflux, leading to nicotinamide adenine dinucleotide (NAD+) depletion in macrophages. Cholesterol-mediated NAD+ depletion induces macrophage senescence, promoting key features of age-related macular degeneration (AMD), including subretinal lipid deposition and neurodegeneration. NAD+ augmentation reverses cellular senescence and macrophage dysfunction, preventing the development of AMD phenotype. Genetic and pharmacological senolysis protect against the development of AMD and neurodegeneration. Subretinal administration of healthy macrophages promotes the clearance of senescent macrophages, reversing the AMD disease burden. Thus, NAD+ deficit induced by excess intracellular cholesterol is the converging mechanism of macrophage senescence and a causal process underlying age-related neurodegeneration.

Roles of Sphingosine Kinase and Sphingosine-1-Phosphate Receptor 2 in Endotoxin-Induced Acute Retinal Inflammation

PURPOSE

To investigate the roles of sphingosine kinases (SphKs) and sphingosine-1-phosphate receptors (S1PRs) in endotoxin-induced uveitis (EIU) mice.

METHODS

EIU model was induced using an intraperitoneal injection of lipopolysaccharide (LPS). The expression of SphKs and S1PRs in the retina was assessed using quantitative polymerase chain reaction (qPCR) and immunofluorescence. The effects of S1PR antagonists on the expression of inflammatory cytokines in the retina were evaluated using qPCR and western blotting. Effects of leukocyte infiltration of the retinal vessels were evaluated to determine the effects of the S1PR2 antagonist and genetic deletion of S1PR2 on retinal inflammation.

RESULTS

Retinal SphK1 expression was significantly upregulated in EIU. SphK1 was expressed in the GCL, IPL, and OPL and S1PR2 was expressed in the GCL, INL, and OPL. Positive cells in IPL and OPL of EIU retina were identified as endothelial cells. S1PR2 antagonist and genetic deletion of S1PR2 significantly suppressed the expression of IL-1α, IL-6, TNF-α, and ICAM-1, whereas S1PR1/3 antagonist did not. Use of S1PR2 antagonist and S1PR2 knockout in mice significantly ameliorated leukocyte adhesion induced by LPS.

CONCLUSION

SphK1/S1P/S1PR2 signaling was upregulated in EIU and S1PR2 inhibition suppressed inflammatory response. Targeting this signaling pathway has potential for treating retinal inflammatory diseases.

Dysregulated CD200-CD200R signaling in early diabetes modulates microglia-mediated retinopathy

Diabetic retinopathy (DR) is a neurovascular complication of diabetes. Recent investigations have suggested that early degeneration of the neuroretina may occur prior to the appearance of microvascular changes; however, the mechanisms underlying this neurodegeneration have been elusive. Microglia are the predominant resident immune cell in the retina and adopt dynamic roles in disease. Here, we show that ablation of retinal microglia ameliorates visual dysfunction and neurodegeneration in a type I diabetes mouse model. We also provide evidence of enhanced microglial contact and engulfment of amacrine cells, ultrastructural modifications, and transcriptome changes that drive inflammation and phagocytosis. We show that CD200-CD200R signaling between amacrine cells and microglia is dysregulated during early DR and that targeting CD200R can attenuate high glucose-induced inflammation and phagocytosis in cultured microglia. Last, we demonstrate that targeting CD200R in vivo can prevent visual dysfunction, microglia activation, and retinal inflammation in the diabetic mouse. These studies provide a molecular framework for the pivotal role that microglia play in early DR pathogenesis and identify a potential immunotherapeutic target for treating DR in patients.

Cytokine profiles in the aqueous humor following brolucizumab administration for exudative age-related macular degeneration

PURPOSE

To identify the inflammatory cytokine profile in the aqueous humor (AH) of patients with intraocular inflammation (IOI) after intravitreal administration of brolucizumab (IVBr) for neovascular age-related macular degeneration.

METHODS

Eight eyes from seven patients with IOI after initial IVBr (IVBrIOI +) were enrolled. Sixteen eyes from 16 patients without IOI after IVBr (IVBrIOI -) and aflibercept (IVA) were used as controls. AH samples were analyzed using a multiplex immunoassay.

RESULTS

C-C motif chemokine ligand (CCL)2, C-X-C motif chemokine ligand (CXCL)1, CXCL10, CXCL13, interleukin (IL)-6, IL-8, IL-10, matrix metalloproteinase (MMP)-1, MMP-9, granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), intercellular adhesion molecule (ICAM)-1, E-selectin, and P-selectin levels were significantly higher in IVBrIOI + than in IVBrIOI - and IVA. Vascular endothelial growth factor (VEGF) was significantly lower in IVBrIOI - compared to that in IVBrIOI + and IVA. In the IVBrIOI + group, there were significant correlations between CCL2, CXCL1, IL-6, IL-8, IL-10, G-CSF, GM-CSF, ICAM-1, and E-selectin, which also exhibited significant correlations in the IVBrIOI - group.

CONCLUSION

The number of inflammatory cytokines increases during IOI, which is associated with type IV hypersensitivity and vascular inflammation. Some cytokines exhibit correlations even in non-inflamed eyes, indicating a subclinical response to IVBr.

Oxidative Stress-Induced Cellular Senescence in Aging Retina and Age-Related Macular Degeneration

Aging leads to a gradual decline of function in multiple organs. Cataract, glaucoma, diabetic retinopathy, and age-related macular degeneration (AMD) are age-related ocular diseases. Because their pathogenesis is unclear, it is challenging to combat age-related diseases. Cellular senescence is a cellular response characterized by cell cycle arrest. Cellular senescence is an important contributor to aging and age-related diseases through the alteration of cellular function and the secretion of senescence-associated secretory phenotypes. As a driver of stress-induced premature senescence, oxidative stress triggers cellular senescence and age-related diseases by inducing senescence markers via reactive oxygen species and mitochondrial dysfunction. In this review, we focused on the mechanism of oxidative stress-induced senescence in retinal cells and its role in the pathogenesis of AMD.

Risk Factors and Treatment Strategy for Retinal Vascular Occlusive Diseases

Retinal occlusive diseases are common diseases that can lead to visual impairment. Retinal artery occlusion and retinal vein occlusion are included in the clinical entity, but they have quite different pathophysiologies. Retinal artery occlusion is an emergent eye disorder. Retinal artery occlusion is mainly caused by thromboembolism, which frequently occurs in conjunction with life-threatening stroke and cardiovascular diseases. Therefore, prompt examinations and interventions for systemic vascular diseases are often necessary for these patients. Retinal vein occlusion is characterized by retinal hemorrhage and ischemia, which may impair visual function via several complications such as macular edema, macular ischemia, vitreous hemorrhage, and neovascular glaucoma. Even though anti-vascular endothelial growth factor therapy is the current established first-line of treatment for retinal vein occlusion, several clinical studies have been performed to identify better treatment protocols and new therapeutic options. In this review, we summarize the current findings and advances in knowledge regarding retinal occlusive diseases, particularly focusing on recent studies, in order to provide an update for a better understanding of its pathogenesis.

Relationship between the vessel density around the optic nerve head and visual field deterioration in eyes with retinitis pigmentosa

PURPOSE

To analyze the vessel density around the optic nerve head (ONH) by optical coherence tomography angiography (OCTA) in patients with retinitis pigmentosa (RP), and to evaluate its correlation with visual functions.

METHODS

Thirty-eight eyes with RP with visual field constriction within the central 10° were enrolled. The mean (± SD) age of the patients was 43.7 ± 15.1 years. In addition to visual acuity (VA) measurements and visual field tests (Humphrey Field Analyzer 10-2 test), we also measured the vessel density at the macula by OCTA (superficial and deep vessel density: sVD(m) and dVD(m)) and in multiple layers around the ONH (vessel density in the radial peripapillary capillary [RPC] layer and in the nerve head [NH] layer: VDrpc and VDnh). The vessel density was calculated by binarizing the OCTA images. The associations between the logMAR VA and mean deviation (MD) values and the variables of central retinal thickness (CRT), sVD(m), dVD(m), VDrpc, VDnh, and also the size of the foveal avascular zone were investigated.

RESULTS

The mean logMAR VA was 0.16 ± 0.34 and the MD value was - 17.2 ± 10.3 dB; the MD value was significantly related to the logMAR VA (p = 0.0028). Multivariate analysis with AICc model selection suggested only dVD(m) was associated with logMAR VA. On the other hand, the optimal model for the MD value included the CRT, dVD(m), and VDnh.

CONCLUSION

The vessel density in the deep layer around the ONH was significantly associated with the visual field deterioration in patients with RP.

OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY STUDY OF CHOROIDAL NEOVASCULARIZATION ASSOCIATED WITH EARLY-ONSET DRUSEN

PURPOSE

To report three middle-aged cases with choroidal neovascularization (CNV) associated with early-onset drusen documented with optical coherence tomography angiography (OCTA).

METHODS

Three patients with bilateral early-onset drusen were referred to our hospital. Fundus examination, fluorescein angiography, indocyanine green angiography, OCTA, and other multimodal imaging were performed.

RESULTS

Case 1 involved a 47-year-old woman who presented with sudden unilateral anorthopia. She had no previous systemic pathologies. Funduscopic examination and fluorescein angiography revealed bilateral large colloid drusen accompanied by unilateral mild subretinal hemorrhage. Indocyanine green angiography revealed CNV, although it was unclear in fluorescein angiography. Optical coherence tomography angiography also showed interconnecting CNV beneath the retinal pigment epithelium. Case 2 involved a 40-year-old woman with membranoproliferative glomerulonephritis Type 3 who presented with unilateral anorthopia. On fluorescein angiography, cuticular drusen secondary to membranoproliferative glomerulonephritis were seen in both eyes. An interconnecting vascular network was revealed with OCTA and indocyanine green angiography indicating Type 1 CNV in the affected eye. Case 3 involved a 47-year-old man without any medical or family history. Predominant large colloid drusen associated with cuticular drusen were seen in both eyes. Unilateral mild serosanguinous changes were accompanied in the macula, where Type 1 CNV was detected with OCTA.

CONCLUSION

All our cases with early-onset drusen showed Type 1 CNV that was detected by indocyanine green angiography or OCTA. Optical coherence tomography angiography has a potential to help noninvasively diagnose CNV in the cases of EOD.

Protective Effects and Molecular Signaling of n-3 Fatty Acids on Oxidative Stress and Inflammation in Retinal Diseases

Oxidative stress and inflammation play crucial roles in the development and progression of retinal diseases. Retinal damage by various etiologies can result in retinopathy of prematurity (ROP), diabetic retinopathy (DR), and age-related macular degeneration (AMD). n-3 fatty acids are essential fatty acids and are necessary for homeostasis. They are important retinal membrane components and are involved in energy storage. n-3 fatty acids also have antioxidant and anti-inflammatory properties, and their suppressive effects against ROP, DR, and AMD have been previously evaluated. α-linolenic acid (ALA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and their metabolites have been shown to alleviate retinal oxidative stress and inflammation involving various biological signaling pathways. In this review, we summarize the current understanding of the n-3 fatty acids effects on the mechanisms of these retinal diseases and how they exert their therapeutic effects, focusing on ALA, EPA, DHA, and their metabolites. This knowledge may provide new remedial strategies for n-3 fatty acids in the prevention and treatment of retinal diseases associated with oxidative stress and inflammation.

Lipid Signaling in Ocular Neovascularization

Vasculogenesis and angiogenesis play a crucial role in embryonic development. Pathological neovascularization in ocular tissues can lead to vision-threatening vascular diseases, including proliferative diabetic retinopathy, retinal vein occlusion, retinopathy of prematurity, choroidal neovascularization, and corneal neovascularization. Neovascularization involves various cellular processes and signaling pathways and is regulated by angiogenic factors such as vascular endothelial growth factor (VEGF) and hypoxia-inducible factor (HIF). Modulating these circuits may represent a promising strategy to treat ocular neovascular diseases. Lipid mediators derived from membrane lipids are abundantly present in most tissues and exert a wide range of biological functions by regulating various signaling pathways. In particular, glycerophospholipids, sphingolipids, and polyunsaturated fatty acids exert potent pro-angiogenic or anti-angiogenic effects, according to the findings of numerous preclinical and clinical studies. In this review, we summarize the current knowledge regarding the regulation of ocular neovascularization by lipid mediators and their metabolites. A better understanding of the effects of lipid signaling in neovascularization may provide novel therapeutic strategies to treat ocular neovascular diseases and other human disorders.

Small Symptomatic Unilateral Choroidal Osteoma Diagnosed with Enhanced-Depth Imaging Optical Coherence Tomography and Thin-Slice Computed Tomography: 2 Clinical Cases

Background

Choroidal osteoma is a rare, benign, ossifying intraocular tumor of unknown etiology. While patients with choroidal osteoma usually show distinct large yellowish subretinal lesions, some could have small lesions, making the differential diagnosis difficult. We experienced 2 cases of small symptomatic unilateral osteoma approximately 1.0-mm disc diameter in size.

Methods

Retrospective medical charts of 2 patients with small symptomatic unilateral osteoma were reviewed. Fundus examination, spectral domain enhanced-depth optical coherence tomography (EDI-OCT), fluorescein angiography, indocyanine green angiography, B-scan ultrasonography (USG), and X-ray computed tomography (CT) were performed.

Results

Case 1: a case of a 41-year-old male. Fundus examination revealed a yellowish-white lesion of 1.0-mm disc diameter in size. EDI-OCT of the lesion shows sub-RPE elevation. B-scan USG was not definite for diagnosis. Thin-slice (2 mm) CT scan revealed a choroidal osteoma. Case 2: a case of a 70-year-old male. Fundus examination revealed a yellowish-white lesion of 0.9-mm disc diameter. EDI-OCT showed sub-RPE elevation without serous retinal detachment and horizontal lamellar-like structure within the lesion. B-scan USG suggested acoustic shadowing, but it was unclear. The thin-slice CT scan confirmed bony tissue at the lesion.

Conclusions

We report 2 cases of small choroidal osteoma. Diagnosis of small yellowish subretinal lesions is sometimes difficult. Characteristic findings with EDI-OCT may sometimes suggest this disease, and thin-slice CT could help to diagnose choroidal osteoma.

Association between pachychoroid and long-term treatment outcomes of photodynamic therapy with intravitreal ranibizumab for polypoidal choroidal vasculopathy

We investigated long-term treatment responses in patients with treatment-naïve polypoidal choroidal vasculopathy (PCV) undergoing photodynamic therapy (PDT) with intravitreal ranibizumab (IVR). The medical charts of 14 patients with treatment-naïve PCV who underwent PDT with IVR were retrospectively reviewed. Patients were followed up and treated with additional IVR for ≥3 years. Best-corrected visual acuity (BCVA), central foveal thickness (CFT), greatest linear dimension (GLD) on angiography, polyp regression and central choroidal thickness (CCT) were assessed. Associations between these functional or anatomic outcomes with age, baseline CCT, baseline GLD or choroidal vascular hyperpermeability (CVH) were investigated using univariate and multivariate analysis. Mean logMAR BCVA improved significantly at 3 years (0.34 ± 0.24 to 0.12 ± 0.29, p = 0.003). Greater BCVA improvement and longer time to first recurrence was significantly associated with CVH. Fewer number of IVR retreatment within 3 years was associated with thicker baseline CCT. Mean CCT significantly decreased at 3 years (217 ± 33 µm to 197 ± 48 µm, p = 0.003). Greater decrease of CCT was significantly associated both with greater number of IVR retreatment within 3 years and absence of CVH. These results showed that pachychoroid characteristics at baseline was associated long-term functional and anatomic outcomes in patients with treatment-naïve PCV who had undergone combination PDT and IVR.

The role of sphingosine 1-phosphate receptors on retinal pigment epithelial cells barrier function and angiogenic effects

Sphingosine-1-phosphate (S1P) is a lysophospholipid mediator, promoting angiogenesis and inflammation via interactions with its receptors (S1P1-5), but the receptors and signaling pathways responsible for the progression of choroidal neovascularization (CNV) remain unknown. We investigated the roles of S1P/S1P receptors in RPE cells. ARPE-19 cells were treated with S1P dissolved in carrier proteins of albumin or apolipoprotein M (ApoM). The mRNA expression levels of interleukin-8 (IL-8), C-C motif chemokine ligand 2 (CCL2), and vascular endothelial growth factor (VEGF) were evaluated using quantitative real-time polymerase chain reaction. The protein level of hypoxia-inducible factor (HIF)-1α was assessed via enzyme-linked immunosorbent assay. HIF transcriptional activity was evaluated with a dual-reporter luciferase assay. Cellular barrier integrity was evaluated using transepithelial electrical resistance and the FITC-dextran permeability assay. The suppressive effect of an S1P antagonist on CNV progression was investigated with a laser-induced CNV model in mice. The increase in expression of IL-8, CCL2, and VEGF due to albumin-bound S1P was significantly mitigated by an S1P2 antagonist. The expression of HIF-1α significantly decreased with inhibition of S1P2 and S1P3. In addition, albumin-bound S1P disrupted the barrier integrity of retinal pigment epithelial cells via S1P2, whereas integrity was strengthened by ApoM-bound S1P. CNV lesions were significantly reduced in the mouse model with intravitreal injection of S1P2 antagonist. This study demonstrated that S1P significantly promotes angiogenesis, inflammation, and barrier integrity, which was attenuated by inhibition of S1P2 or S1P3, suggesting that regulation of S1P2 and S1P3 is a novel therapeutic target for CNV.

Light Stress-Induced Increase of Sphingosine 1-Phosphate in Photoreceptors and Its Relevance to Retinal Degeneration

Sphingosine 1-phosphate (S1P) is a potent lipid mediator that modulates inflammation and angiogenesis. In this study, we investigated the possible involvement of S1P in the pathology of light-induced retinal degeneration in vivo and in vitro. The intracellular S1P and sphingosine kinase (SphK) activity in a photoreceptor cell line (661W cells) was significantly increased by exposure to light. The enhancement of SphK1 expression was dependent on illumination, and all-trans-retinal significantly promoted SphK1 expression. S1P treatment reduced protein kinase B (Akt) phosphorylation and increased the protein expression of cleaved caspase-3, and induced photoreceptor cell apoptosis. In vivo, light exposure enhanced the expression of SphK1 in the outer segments of photoreceptors. Intravitreal injection of a SphK inhibitor significantly suppressed the thinning of the outer nuclear layer and ameliorated the attenuation of the amplitudes of a-waves and b-waves of electroretinograms during light-induced retinal degeneration. These findings imply that light exposure induces the synthesis of S1P in photoreceptors by upregulating SphK1, which is facilitated by all-trans-retinal, causing retinal degeneration. Inhibition of this enhancement may be a therapeutic target of outer retinal degeneration, including age-related macular degeneration.

Kupffer cell targeting by intraportal injection of the HVJ cationic liposome

The aim of this study was to target Kupffer cells (KCs) selectively and efficiently by the intraportal injection of fusigenic cationic liposomes with hemagglutinating virus of Japan components (HVJ cationic liposomes). Phosphorothioate FITC-oligodeoxynucleotides (FITC-ODNs) encapsulated in either HVJ cationic liposomes, HVJ anionic liposomes or conventional cationic liposomes without HVJ were transferred to the rat. FITC-ODNs in HVJ cationic liposomes administered via portal vein were selectively transfected to KCs for up to 24 h with no apparent cytotoxicity at higher transfection efficiency than FITC-ODNs in conventional cationic liposomes without HVJ administered via portal vein or tail vein. On the other hand, FITC-ODNs in HVJ anionic liposomes were observed mainly in hepatocytes, not KCs. This new method will be useful for the modulation of KCs activity in both basic research and clinical applications.

Gene transfer to the rat biliary tract with the HVJ-cationic liposome method

BACKGROUND/AIMS

The ability to transfer foreign genes into the biliary tract would be useful for the treatment of biliary tract diseases, including cancer, cystic fibrosis and other genetic diseases. To introduce a foreign gene precisely into the rat biliary epithelial cells, we developed a new technique, inserting a polyethylene catheter into the common bile duct through the papilla of Vater by use of a fusigenic cationic liposome with hemagglutinating virus of Japan (HVJ-cationic liposome).

METHODS

Transfection efficiency was estimated with the use of FITC-oligonucleotides (FITC-ODNs) and cDNA of beta-galactosidase (pCAG-lacZ).

RESULTS

FITC-ODNs encapsulated in HVJ-cationic liposome were effectively transfected into cell nuclei of human cholangiocellular carcinoma in vitro after a 30-min incubation as compared with the simple application of naked FITC-ODNs. After in vivo injection of FITC-ODNs using the HVJ-cationic liposome method through the papilla of Vater, fluorescence accumulation was observed only in the epithelial cells of the biliary tract, but not in the parenchymal cells of the liver. Beta-galactosidase expression was observed in the biliary epithelial cells 3 days after the transfection of pCAG-lacZ and was also detected at 14 days, but not at 28 days, without obvious cytotoxicity.

CONCLUSIONS

HVJ-cationic liposome-mediated gene transfer to the biliary tract via the papilla of Vater is a minimally-invasive and an effective gene-delivery method for site-specific targeting to the epithelial cells of the biliary tract, which could be applied to the treatment of human biliary tract diseases.

Suppression of proliferative cholangitis in a rat model with direct adenovirus-mediated retinoblastoma gene transfer to the biliary tract

Proliferative cholangitis (PC) associated with hepatolithiasis develops the stricture of main bile ducts, and is the main cause of residual and/or recurrent stones after repeated treatments for hepatolithiasis. The aim of this study was to inhibit PC using the cytostatic gene therapy with direct adenovirus-mediated retinoblastoma (Rb) gene transfer to the biliary tract. PC was induced by introducing a fine nylon thread into the bile duct in a rat model. The adenovirus vector encoding a nonphosphorylatable, constitutively active form of retinoblastoma gene product (AdRb) was administered directly into the biliary tract. The adenovirus vector encoding beta-galactosidase (AdlacZ) was also given as a control. The bile duct wall thickness and 5'-bromodeoxyuridine (BrdU) labeling index were compared among uninfected, AdlacZ-infected, and AdRb-infected PC rats. The Rb expression in the bile duct was detected using reverse-transcription polymerase chain reaction (RT-PCR) and immunohistochemical study. AdRb-infected bile ducts showed inhibition of the epithelial and fibrous tissue proliferation and the peribiliary gland hyperplasia, resulting in a significant reduction of wall thickness compared with uninfected and AdlacZ-infected ones. The BrdU labeling index was 4.87% +/- 3.06% in the AdRb-infected bile ducts, while those of uninfected and AdlacZ-infected ones were 15.48% +/- 4.61% and 11.72% +/- 1.23%, respectively (P < .05). In conclusion, our cytostatic gene therapy approach using direct Rb gene transfer into the biliary tract suppressed PC in a rat model and may offer an effective therapeutic option for reducing recurrences following treatments against hepatolithiasis.

Four subclones with distinct immunoglobulin light chain phenotypes. (kappa+lambda+, kappa+, lambda+ and kappa-lambda-) from acute leukemia

A human acute lymphoblastic leukemia (ALL) cell line, BALM-9, was established from the peripheral blood specimen of a immunoglobin (lg) phenotype, the established BALM-9 cell line expressed both kappa and lambda light (L) chains simultaneously in a range of 30-80%. Two-color flow cytometric analysis demonstrated that there was a distinct population of kappa lambda double positive cells as well as kappa single, lambda single, and double negative populations. Therefore, subclones were obtained from each population by limiting dilution and were designated BALM-9KL (kappa+lambda+), BALM-9K (kappa+lambda-), BALM 9N (kappa-lambda-). Western blotting confirmed the results of the immunofluorescence test at the protein level. In BALM-9N, L chains were absent even in the cytoplasm as demonstrated by Western blotting. Evidence that the subclones have the same ancestry was provided both by cytogenetic analysis and by Southern blotting, which revealed the 14q32 chromosomal rearrangement as a common abnormality and the same IgH gene arrangement among the subclones. The existence of a kappa lambda positive B cell population suggests a transient stage of normal B cell maturation. These subclones might represent such a stage and thus provide a useful means of analyzing the mechanism of this double light chain expression.

Immunological studies on delayed hypersensitivity to phenytoin--I. A new method for preparing phenytoin antiserum

The present study was initiated to produce an antiserum to phenytoin with high specificity and sensitivity which would be suitable not only for determination of blood phenytoin concentration but also for induction of a hypersensitivity reaction to phenytoin in experimental animals. p-Aminophenytoin was synthesized and identified by means of IR, 1H-NMR and mass spectroscopy. BSA-phenytoin conjugate was prepared by using p-aminophenytoin, BSA and, as a coupling reagent, glutaraldehyde. Satisfactory response to immunization was achieved at a 9.8:1 molar ratio of p-aminophenytoin to BSA. The antiserum obtained from rabbits immunized with BSA-phenytoin conjugate exhibited practically no cross-reactivity with either phenytoin metabolites or other anti-epileptic drugs, indicating that this antiserum provides sufficiently high specificity. In our experiments, the lower limit for detecting phenytoin was 2 ng using RIA, whereas 200 ng was the minimum amount detectable by HPLC. Thus, by a difference of two orders of magnitude, the present RIA method shows a much higher sensitivity than that of HPLC, though we found a good correlation of simultaneous determinations of serum phenytoin between the two methods. Reproducibility of phenytoin determination in plasma was confirmed by calculating the coefficient of variance. The values were less than 10%.