Perspectives on long-acting formulations of pre-exposure prophylaxis (PrEP) among men who have sex with men who are non-adherent to daily oral PrEP in the United States

INTRODUCTION

Pre-exposure prophylaxis (PrEP) persistence among men who have sex with men (MSM) in real world clinical settings for HIV prevention is suboptimal. New longer-acting formulations of PrEP are becoming available, including injectables, subdermal implants, and other oral medications. These longer-acting formulations have the potential to improve retention among those who have challenges remaining adherent to daily oral PrEP.

METHODS

We interviewed 49 MSM who had initiated but discontinued oral PrEP at three diverse clinics across the United States. We examined participants' perspectives about long-acting PrEP formulations and how long-acting options could affect PrEP use using thematic analysis.

RESULTS

Participants were not very knowledgeable about long-acting formulations of PrEP but were open to learning about them and considering use. Participants were concerned about safety and efficacy of products given that they were still newer and/or in development. Finally, participants had clear preferences for oral pills, injectables, and then subdermal implants and were most interested in options that reduced the number of visits to the clinic.

CONCLUSION

Long-acting formulations of PrEP are acceptable to MSM with suboptimal PrEP persistence and have the potential to improve PrEP persistence. However, many felt they needed more information on safety, efficacy, and use to consider these options. As these long-acting formulations are implemented, public health campaigns and clinical interventions to encourage may maximize uptake particularly among those who are not currently adherent to daily oral PrEP.

Abstract OT3-05-01: Adaptive Multi-Drug Treatment of Evolving Cancers (AMTEC): A Phase II, Open-Label, Study of Olaparib in Combination with either Durvalumab, Selumetinib or Capivasertib, or Ceralasertib Monotherapy in Patients with Metastatic TNBC

BACKGROUND PARP inhibitors (PARPi) afford a rational therapeutic strategy in metastatic TNBC (mTNBC) due to the high incidence of dysregulated DNA damage repair mechanisms and high-level genomic instability that resemble tumors originating in germline BRCA-mutated carriers. However, PARPi monotherapy has limited efficacy in BRCA wild-type mTNBC; in BRCA mutant disease following initial response, compensatory mechanisms inevitably restore replication fork protection. AMTEC leverages pre- and on-therapy biopsies from a 4-week PARPi monotherapy run-in period for personalized biomarker-driven patient selection to interdict adaptive resistance to the PARPi. Data from our pilot study (NCT03544125) and from Arm 1 of AMTEC (olaparib + durvalumab) identified PI3K-AKT, RAS-MEK, and ATR/CHK1/WEE1 as targetable pathways contributing to PARPi adaptive resistance in individual participants. Clinically validated assays (DNA, RNA, and protein) enable the identification of cellular mechanisms of PARPi sensitivity and resistance in individual patients and further reveal combined drug treatments that could prevent emergence of PARPi resistance. METHODS AMTEC is a non-comparative, multi-arm, open-label, phase II study to assess the efficacy of combining olaparib (ola) with durvalumab (dur), or MEKi, selumetinib (sel), or AKTi, capivasertib (cap), or monotherapy with ATRi, ceralasertib, (cer mono) in mTNBC patients. Participants with biopsy proven mTNBC (ER< 10%, PR< 10%, and HER-2 non-amplified), AR< 80% are eligible. - Participants undergo a pre-treatment biopsy, then start a 28-day induction with ola (300 mg PO BID, D1-28). On C1D14, patients undergo a repeat, on-treatment biopsy. Clinically validated assays (DNA, RNA, protein) from both biopsies inform patient assignment to a specific ola combination arm starting on C2D1: - Arm 1 tumor immune activated: ola + dur (1500 mg IV Q4W) - Arm 2 RAS-MEK-ERK pathway activation: ola + sel (BSA-based BID D1-28) - Arm 3 PI3K-AKT pathway activation: ola + cap (400 mg PO BID, 4 days on/3 days off) - Arm 4: If not eligible for Arms 1-3 (per biomarker selection criteria): Cer mono (240 mg PO BID D1-14) Endpoints: The primary endpoint is objective response rate (ORR per RECIST 1.1). Secondary endpoints include safety and toxicity, clinical benefit rate, duration of response, and survival. Statistical Methods: - Arm 1 will enroll 28 patients to detect an ORR difference of 20% (H0: π = 0.15 and Ha: π =0.35). Arm 1 will continue on to stage 2 if ORR ≥3 of the first 15 patients. The null hypothesis for Arm 1 is rejected if ≥ 7/28 patient achieve a response. - Arms 2, 3, and 4, will each enroll 22 patients to detect an ORR difference of 25% (H0: π = 0.15 and Ha: π =0.40). Arms 2, 3, and 4 will each continue on to stage 2 if ORR ≥2 of first 11 patients in each arm, respectively. The null hypothesis for Arm 2, 3, and 4 is rejected if ≥ 7/22 patients achieve a response in each arm, respectively. For arms 2 and 3, if there are ≥5/11 responses, the trial will open a biomarker negative expansion cohort for each arm (N = 19 patients/arm). ENROLLMENT The study was activated on 1/7/2019. Arm 1 met pre-specified interim analysis criteria in 12/2020, and accrual to stage 2 began in 1/2021. Arms 2, 3, and 4 start enrolling in Q4 of 2022. Up to 132 patients will be enrolled. Clinical trial information: NCT03801369 Contact information: For more information or to refer a patient, email hobbev@ohsu.edu

Citation Format: Evthokia Hobbs, Gordon Mills, Jeong Lim, Marlana Klinger, Kiara Siex, Sidney Huszti, Annie Yang, Danielle Galipeau, Christina Zheng, Lauren Murray, Becky Goodford, Nicholas Marter-Sanders, Anastasiya Olson, Jayne Stommel, Brett Johnson, Jamie Keck, Ben Kong, Allison Solanki, Shaun Goodyear, Christopher Corless, Joe Gray, Mitri Zahi. Adaptive Multi-Drug Treatment of Evolving Cancers (AMTEC): A Phase II, Open-Label, Study of Olaparib in Combination with either Durvalumab, Selumetinib or Capivasertib, or Ceralasertib Monotherapy in Patients with Metastatic TNBC [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT3-05-01.

A window of opportunity trial for metastatic (WOO-M) pancreatic ductal adenocarcinoma (PDAC): A biomarker discovery platform

TPS781

Background: Even as the number of targeted agents increases, therapeutic options for PDAC patients remain limited by a lack of biomarkers that can predict the potential clinical benefit of single agents or combination therapeutic strategies. To address this challenge, we implemented the WOO-M trial to serve as a highly adaptable, biomarker discovery platform that enables a preliminary assessment of biological activity of one or more targeted agents in patients with metastatic (m) PDAC. By using paired pre- and on-treatment biopsies, we can assess the independent biological impact of targeted agent(s) that perturb key signaling pathways predicted to drive PDAC tumorigenesis and resistance to therapy in order to identify new combination therapy strategies. Methods: WOO-M is a multi-arm early phase I trial platform to obtain longitudinal tumor samples to assess the pharmacodynamic (PD) effects of one or more study agent(s) alone or in combination in patients with mPDAC. The trial uses a Master Protocol to describe overarching design and logistics, and sub-protocols separately describe each study arm with different agent(s) – alone or in combination. Participants are alternatingly assigned to an available study arm (max. 20 participants per arm). There are no limits to the number of prior therapies. Participants undergo a baseline tumor biopsy, then receive their assigned study agent(s) for a specified timeframe (not exceeding 30 days), and then undergo a repeat tumor biopsy before proceeding to receive therapy per standard of care or clinical trials. The paired biopsy material from each patient is analyzed using multiplex assays that provides spatially-resolved, single-cell strategies to assess the effects of targeted therapies on tumor cell state and heterogeneity. The primary study objective is to independently assess the PD feasibility of detecting a measurable change in tumor biology at post-treatment from baseline for participants within a study arm. WOO-M uses a 2-stage Bayesian efficacy monitoring approach with a futility-stopping rule for each study arm. In stage 1, if 6 or more of the first 10 participants of a study arm have a detectable change in tumor biology measurements post-treatment from baseline, then the study arm may enroll up to an additional 10 participants. A study arm stops enrolling if 5 or fewer of the first 10 participants do not have a measurable change in tumor biology. To date, 4 study arms are being evaluated: 1) poly (ADP-ribose) polymerase inhibitor (PARPi), olaparib (300 mg PO BID for 10 days), 2) MEK inhibitor (MEKi), cobimetinib (60 mg PO QD for 10 days), 3) ERK inhibitor, LY3214996 (400 mg PO QD for 10 days), and 4) PLK1 inhibitor, onvansertib (12 mg/m2 PO QD for 10 days). Of 21 participants enrolled and treated to date, 8 received cobimetinib, and 13 received olaparib. Additional arms are in the process of opening at time of submission. Clinical trial information: NCT04005690

Machine learning for rhabdomyosarcoma histopathology

Correctly diagnosing a rare childhood cancer such as sarcoma can be critical to assigning the correct treatment regimen. With a finite number of pathologists worldwide specializing in pediatric/young adult sarcoma histopathology, access to expert differential diagnosis early in case assessment is limited for many global regions. The lack of highly-trained sarcoma pathologists is especially pronounced in low to middle-income countries, where pathology expertise may be limited despite a similar rate of sarcoma incidence. To address this issue in part, we developed a deep learning convolutional neural network (CNN)-based differential diagnosis system to act as a pre-pathologist screening tool that quantifies diagnosis likelihood amongst trained soft-tissue sarcoma subtypes based on whole histopathology tissue slides. The CNN model is trained on a cohort of 424 centrally-reviewed histopathology tissue slides of alveolar rhabdomyosarcoma, embryonal rhabdomyosarcoma and clear-cell sarcoma tumors, all initially diagnosed at the originating institution and subsequently validated by central review. This CNN model was able to accurately classify the withheld testing cohort with resulting receiver operating characteristic (ROC) area under curve (AUC) values above 0.889 for all tested sarcoma subtypes. We subsequently used the CNN model to classify an externally-sourced cohort of human alveolar and embryonal rhabdomyosarcoma samples and a cohort of 318 histopathology tissue sections from genetically engineered mouse models of rhabdomyosarcoma. Finally, we investigated the overall robustness of the trained CNN model with respect to histopathological variations such as anaplasia, and classification outcomes on histopathology slides from untrained disease models. Overall positive results from our validation studies coupled with the limited worldwide availability of sarcoma pathology expertise suggests the potential of machine learning to assist local pathologists in quickly narrowing the differential diagnosis of sarcoma subtype in children, adolescents, and young adults.

Facilitators for retaining men who have sex with men in pre-exposure prophylaxis care in real world clinic settings within the United States

BACKGROUND

Pre-exposure prophylaxis (PrEP) can significantly reduce HIV acquisition especially among communities with high HIV prevalence, including men who have sex with men (MSM). Much research has been finding suboptimal PrEP persistence; however, few studies examine factors that enhance PrEP persistence in real-world settings.

METHODS

We interviewed 33 patients who identified as MSM at three different PrEP clinics in three regions of the U.S. (Northeast, South, Midwest). Participants were eligible if they took PrEP and had been retained in care for a minimum of 6 months. Interviews explored social, structural, clinic-level and behavioral factors that influencing PrEP persistence.

RESULTS

Through thematic analysis we identified the following factors as promoting PrEP persistence: (1) navigation to reduce out-of-pocket costs of PrEP (structural), (2) social norms that support PrEP use (social), (3) access to LGBTQ + affirming medical providers (clinical), (4) medication as part of a daily routine (behavioral), and (5) facilitation of sexual health agency (belief).

DISCUSSION

In this sample, persistence in PrEP care was associated with structural and social supports as well as a high level of perceived internal control over protecting their health by taking PrEP. Patients might benefit from increased access, LGBTQ + affirming medical providers, and communications that emphasize PrEP can promote sexual health.

Effects of lipoic acid on primary murine microglial cells

The anti-oxidant lipoic acid (LA) is beneficial in murine models of multiple sclerosis (MS) and has recently been shown to slow brain atrophy in secondary progressive MS. The mechanism of these effects by LA is incompletely understood but may involve effects on microglia. The objective of this study is to understand how LA affects microglial cells. We cultured primary microglial cells from C57BL/6 adult mice brains and stimulated the cells with lipopolysaccharide (LPS) and interferon gamma (IFN-γ) in the presence or absence of LA. We demonstrate the inhibition of phagocytosis, rearrangement of actin, and formation of membrane blebs in stimulated microglia in the presence of LA. These experiments suggest that LA causes changes in microglial actin, which may lead to alterations in phagocytosis, mobility, and migration.

Myelin repair stimulated by CNS-selective thyroid hormone action

Oligodendrocyte processes wrap axons to form neuroprotective myelin sheaths, and damage to myelin in disorders, such as multiple sclerosis (MS), leads to neurodegeneration and disability. There are currently no approved treatments for MS that stimulate myelin repair. During development, thyroid hormone (TH) promotes myelination through enhancing oligodendrocyte differentiation; however, TH itself is unsuitable as a remyelination therapy due to adverse systemic effects. This problem is overcome with selective TH agonists, sobetirome and a CNS-selective prodrug of sobetirome called Sob-AM2. We show here that TH and sobetirome stimulated remyelination in standard gliotoxin models of demyelination. We then utilized a genetic mouse model of demyelination and remyelination, in which we employed motor function tests, histology, and MRI to demonstrate that chronic treatment with sobetirome or Sob-AM2 leads to significant improvement in both clinical signs and remyelination. In contrast, chronic treatment with TH in this model inhibited the endogenous myelin repair and exacerbated disease. These results support the clinical investigation of selective CNS-penetrating TH agonists, but not TH, for myelin repair.

Lipoic acid reduces inflammation in a mouse focal cortical experimental autoimmune encephalomyelitis model

Cortical lesions are a crucial part of MS pathology and it is critical to determine that new MS therapies have the ability to alter cortical inflammatory lesions given the differences between white and gray matter lesions. We tested lipoic acid (LA) in a mouse focal cortical EAE model. Brain sections were stained with antibodies against CD4, CD11b and galectin-3. Compared with vehicle, treatment with LA significantly decreased CD4+ and galectin-3+ immune cells in the brain. LA treated mice had fewer galectin-3+ cells with no projections indicating decrease in the number of infiltrating monocytes. LA significantly reduces inflammation in a focal cortical model of MS.

Mechanism of the enhancement of electrical conductivity of nanocrystalline silicon due to hydrogen plasma treatment

The mechanism of electrical charge transport in hydrogenated nanocrystalline silicon (nc-Si:H) and the enhancement in electrical conductivity by hydrogen plasma exposure has been studied. Nanoscale electrical conduction measurements (laterally on the surface) suggested that the dominant charge transport in nc-Si:H occurs through the crystalline grain interiors while grain boundaries are highly resistive. Room temperature low-power/short-duration (10 W, 10 s) surface hydrogen plasma treatment enhanced the local surface and bulk electrical conductivity of nc-Si:H films which was attributed to improved passivation of surface and bulk dangling bonds, increase in crystalline fraction and decrease in grain boundary (GB) fraction. However, the improvement in electrical conductivity due to high-power/long-duration (50 W, 10 min) hydrogen plasma exposure was not as pronounced as low-power/short-duration exposure. Temperature-dependent dark conductivity measurements showed dual activation-energy behavior; increase in activation energy in the high-temperature regime (400-585 K) was attributed to the temperature dependence of tunneling probability of carriers and explained using a heteroquantum dots model. A decrease in activation energy with plasma exposure was observed which was explained using the framework of a three-phase model of nc-Si:H where GB width and barrier potential played a critical role in determining the relative contribution of tunneling and thermally activated carrier transport.

PD-1 Interaction with PD-L1 but not PD-L2 on B-cells Mediates Protective Effects of Estrogen against EAE

Increased remissions in multiple sclerosis (MS) during late pregnancy may result from high levels of sex steroids such as estrogen and estriol. Estrogen (E2=17β-estradiol) protects against experimental autoimmune encephalomyelitis (EAE), but the cellular basis for E2-induced protection remains unclear. Treatment with relatively low doses of E2 can protect against clinical and histological signs of MOG-_35-55 induced EAE through mechanisms involving the PD-1 coinhibitory pathway and B-cells. The current study evaluated the contribution of PD-1 ligands, PD-L1 and PD-L2, on B-cells in E2-mediated protection against EAE in WT, PD-L1^-/- and PD-L2^-/- mice. Unlike PD-L2^-/- mice that were fully protected against EAE after E2 treatment, E2-implanted PD-L1^-/- mice were fully susceptible to EAE, with increased numbers of proliferating Th1/Th17 cells in the periphery and severe cellular infiltration and demyelination in the CNS. Moreover, transfer of B-cells from MOG-immunized PD-L1^-/- or PD-L2^-/- donors into E2-preconditioned B-cell deficient μMT^-/- recipient mice revealed significantly reduced E2-mediated protection against EAE in recipients of PD-L1^-/- B-cells, but near-complete protection in recipients of PD-L2^-/- B-cells. We conclude that PD-1 interaction with PD-L1 but not PD-L2 on B-cells is crucial for E2-mediated protection in EAE and that strategies that enhance PD-1/PD-L1 interactions might potentiate E2 treatment effects in MS.

Genetic inactivation of the p66 isoform of ShcA is neuroprotective in a murine model of multiple sclerosis

Although multiple sclerosis (MS) has traditionally been considered to be an inflammatory disease, recent evidence has brought neurodegeneration into the spotlight, suggesting that accumulated damage and loss of axons is critical to disease progression and the associated irreversible disability. Proposed mechanisms of axonal degeneration in MS posit cytosolic and subsequent mitochondrial Ca(2+) overload, accumulation of pathologic reactive oxygen species (ROS), and mitochondrial dysfunction leading to cell death. In this context, the role of the p66 isoform of ShcA protein (p66) may be significant. The ShcA isoform is uniquely targeted to the mitochondrial intermembrane space in response to elevated oxidative stress, and serves as a redox enzyme amplifying ROS generation in a positive feedforward loop that eventually mediates cell death by activation of the mitochondrial permeability transition pore. Consequently, we tested the hypothesis that genetic inactivation of p66 would reduce axonal injury in a murine model of MS, experimental autoimmune encephalomyelitis (EAE). As predicted, the p66-knockout (p66-KO) mice developed typical signs of EAE, but had less severe clinical impairment and paralysis than wild-type (WT) mice. Histologic examination of spinal cords and optic nerves showed significant axonal protection in the p66-KO tissue, despite similar levels of inflammation. Furthermore, cultured p66-KO neurons treated with agents implicated in MS neurodegenerative pathways showed greater viability than WT neurons. These results confirm the critical role of ROS-mediated mitochondrial dysfunction in the axonal loss that accompanies EAE, and identify p66 as a new pharmacologic target for MS neuroprotective therapeutics.

Lipoic acid decreases inflammation and confers neuroprotection in experimental autoimmune optic neuritis

Lipoic acid (LA) is an antioxidant that is effective in treating experimental autoimmune encephalomyelitis (EAE), a model for multiple sclerosis (MS). C57BL/6 mice with EAE develop experimental autoimmune optic neuritis (EAON), which models acute optic neuritis in humans. Here we determined whether LA is therapeutically effective in EAON. We immunized C57BL/6 mice with MOG 35-55 peptide. Mice received either daily subcutaneous injections of LA (100mg/kg) or saline in early or late suppression paradigms. In the early suppression paradigm, optic nerve cross-sections showed 14.9±3.8% (mean±SEM) damage in mice receiving saline (n=7) and 2.0±0.4% damage in mice given LA (n=7, p=0.001). In the late suppression paradigm, optic nerve sections showed 24.6±3.5% damage in mice treated with saline (n=7) and 8.4±2.5% in mice treated with LA (n=7, p=0.004). Thus a dramatic reduction in axonal injury was seen after LA administration in both experimental paradigms. Compared with saline treated mice with EAON, optic nerves from mice receiving LA had significantly fewer CD4+ and CD11b+ cells in both paradigms. This study provides a rationale for investigating the therapeutic efficacy of LA in acute optic neuritis in humans.

Chronic thromboxane synthase inhibition prevents fructose-induced hypertension

To investigate the role of thromboxane A(2) in the development of hypertension in the fructose-fed rat, we treated male fructose-fed rats with dazmegrel (a thromboxane synthase inhibitor) and monitored blood pressure, fasting plasma parameters, and insulin sensitivity for 7 weeks. Systolic blood pressure was measured each week using tail plethysmography, and an oral glucose tolerance test was performed at the end of the study to assess insulin sensitivity. Treatment with a 60% fructose diet and dazmegrel (100 mg. kg(-1). d(-1) via oral gavage) was initiated on the same day. Plasma triglyceride levels increased 2-fold in both fructose- and fructose/dazmegrel-treated groups, and plasma insulin levels tended to be higher in these groups, although not significantly. Systolic blood pressure increased significantly throughout the study in the fructose-fed group only (132+/-3 versus 112+/-4 mm Hg in control rats, 118+/-2 mm Hg in control-treated rats, 116+/-2 mm Hg in fructose-treated rats). Both fructose groups demonstrated a higher peak insulin response to oral glucose challenge and had 40% to 60% lower insulin sensitivity index values. The results of this study show that treatment with a thromboxane synthase inhibitor, dazmegrel, can prevent the development of hypertension but does not improve insulin sensitivity or other fructose-induced metabolic impairments. Based on these data, we conclude that the potent vasoconstrictor thromboxane is involved in the link between hyperinsulinemia/insulin resistance and hypertension.

NO/PGI2-independent vasorelaxation and the cytochrome P450 pathway in rabbit carotid artery

1. The nature and cellular mechanisms that are responsible for endothelium-dependent relaxations resistant to indomethacin and NG-nitro-L-arginine methyl ester (L-NAME) were investigated in phenylephrine (PE) precontracted isolated carotid arteries from the rabbit. 2. In the presence of the cyclo-oxygenase inhibitor, indomethacin (10 microM), acetylcholine (ACh) induced a concentration- and endothelium-dependent relaxation of PE-induced tone which was more potent than the calcium ionophore A23187 with pD2 values of 7.03 +/- 0.12 (n = 8) and 6.37 +/- 0.12 (n = 6), respectively. The ACh-induced response was abolished by removal of the endothelium, but was not altered when indomethacin was omitted (pD2 value 7.00 +/- 0.10 and maximal relaxation 99 +/- 3%, n = 6). Bradykinin and histamine (0.01-100 microM) had no effect either upon resting or PE-induced tone (n = 5). 3. In the presence of indomethacin plus the NO synthase inhibitor, L-NAME (30 microM), the response to A23187 was abolished. However, the response to ACh was not abolished, although it was significantly inhibited with the pD2 value and the maximal relaxation decreasing to 6.48 +/- 0.10 and 67 +/- 3%, respectively (for both P < 0.01, n = 8). The L-NAME/indomethacin insensitive vasorelaxation to ACh was completely abolished by preconstriction of the tissues with potassium chloride (40 mM, n = 8). 4. The Ca(2+)-activated K+ (KCa) channel blockers, tetrabutylammonium (TBA, 1 mM, n = 5) and charybdotoxin (CTX, 0.1 microM, n = 5), completely inhibited the nitric oxide (NO) and prostacyclin (PGI2)-independent relaxation response to ACh. However, iberiotoxin (ITX, 0.1 microM, n = 8) or apamin (1-3 microM, n = 6) only partially inhibited the relaxation. 5. Inhibitors of the cytochrome P450 mono-oxygenase, SKF-525A (1-10 microM, n = 6), clotrimazole (1 microM, n = 5) and 17-octadecynoic acid (17-ODYA, 3 microM, n = 7) also reduced the NO/PGI2-independent relaxation response to ACh. 6. In endothelium-denuded rings of rabbit carotid arteries, the relaxation response to exogenous NO was not altered by either KCa channel blockade with apamin (1 microM, n = 5) or CTX (0.1 microM, n = 5), or by the cytochrome P450 mono-oxygenase blockers SKF-525A (10 microM, n = 4) and clotrimazole (10 microM, n = 5). However, the NO-induced response was shifted to the right by LY83583 (10 microM, n = 4), a guanylyl cyclase inhibitor, with the pD2 value decreasing from 6.95 +/- 0.14 to 6.04 +/- 0.09 (P < 0.01). 7. ACh (0.01-100 microM) induced a concentration-dependent relaxation of PE-induced tone in endothelium-denuded arterial segments sandwiched with endothelium-intact donor segments. This relaxation to ACh was largely unaffected by indomathacin (10 microM) plus L-NAME (30 microM), but abolished by the combination of indomethacin, L-NAME and TBA (1 mM, n = 5). 8. These data suggest that in the rabbit carotid artery: (a) ACh can induce the release of both NO and EDHF, whereas A23187 only evokes the release of NO from the endothelium, (b) the diffusible EDHF released by ACh may be a cytochrome P450-derived arachidonic acid metabolite, and (c) EDHF-induced relaxation involves the opening of at least two types of KCa channels, whereas NO mediates vasorelaxation via a guanosine 3': 5'-cyclic monophosphate (cyclic GMP)-mediated pathway, in which a cytochrome P450 pathway and KCa channels do not seem to be involved.