VIP

Comparison with PDE5 Inhibitors

VIP-based intracavernosal therapy occupies a second-line treatment niche for patients who fail oral PDE5 inhibitors (sildenafil, tadalafil, vardenafil). Because VIP acts through the cAMP pathway rather than the cGMP/NO pathway targeted by PDE5 inhibitors, it provides an alternative mechanism that can be effective in patients with severe endothelial dysfunction or nerve damage where NO signaling is compromised (Gerstenberg et al., 1992).

Neurogenic Erectile Dysfunction

VIP-based therapy has shown particular promise in neurogenic ED, including post-radical prostatectomy and spinal cord injury patients. Because VIP does not depend on intact nitrergic nerve signaling — acting instead on smooth muscle VIP receptors directly — it retains efficacy when NO-producing nerve fibers are damaged or absent. Dinsmore & Alderdice (1993) reported favorable responses to VIP/phentolamine in men with spinal cord injuries refractory to papaverine monotherapy.

VIP as NANC Neurotransmitter in Corpus Cavernosum

Immunohistochemical studies have identified dense VIP-immunoreactive nerve fibers surrounding the smooth muscle of the corpus cavernosum, helicine arteries, and cavernosal veins (Polak et al., 1981). Electrical field stimulation of human corporal tissue strips produces relaxation that is partially resistant to NO synthase inhibitors (L-NAME) but blocked by VIP receptor antagonists, confirming VIP's role as a NANC neurotransmitter independent of NO. Men with erectile dysfunction show reduced VIP-containing nerve fiber density compared to potent controls, suggesting that VIP depletion may contribute to ED pathophysiology.

Circadian Rhythm and the SCN

VIP is the principal synchronizing signal within the SCN circadian clock. Aton et al. (2005) demonstrated that VIP-deficient (VIP-/-) mice show profoundly disrupted circadian rhythms: individual SCN neurons continue to oscillate but at desynchronized phases and periods, producing an arrhythmic behavioral phenotype Aton et al. (2005). VPAC2-deficient mice show an identical phenotype, confirming that VIP acts through VPAC2 for circadian coupling.

VIP neurons in the ventral SCN receive direct retinal input via the retinohypothalamic tract and relay photic information to dorsal SCN neurons via VIP/VPAC2 paracrine signaling. This positions VIP as a critical intermediary between environmental light signals and the molecular clockwork. VIP also mediates SCN communication with downstream target regions (subparaventricular zone, dorsomedial hypothalamus), influencing sleep-wake cycles, body temperature rhythms, and hormone release patterns. The SCN VIP system interacts with PACAP (co-released from retinal afferents) to fine-tune photic entrainment.

Hippocampal Synaptic Plasticity

VIP modulates hippocampal long-term potentiation (LTP) through VPAC1-mediated enhancement of glutamatergic transmission and VPAC2-mediated modulation of inhibitory circuits. VIP interneurons in the hippocampus (particularly in CA1 stratum oriens/alveus) form interneuron-selective circuits that disinhibit pyramidal neurons during active behavioral states, facilitating learning and memory encoding. Disruption of hippocampal VIP signaling impairs spatial memory and contextual fear conditioning.

Schizophrenia

Multiple lines of evidence implicate disrupted VIP signaling in schizophrenia pathophysiology. Post-mortem studies have reported reduced VIP-immunoreactive neurons and VIP peptide levels in the prefrontal cortex and superior temporal gyrus of schizophrenia patients Rao et al. (2013). Given that VIP interneurons form a critical component of cortical disinhibitory circuits controlling sensory processing and attention, loss of VIP interneuron function could contribute to the sensory gating deficits and cognitive impairments characteristic of schizophrenia.

VIP interneuron dysfunction may also contribute to the circadian rhythm disruptions frequently observed in schizophrenia — disrupted sleep-wake cycles, abnormal melatonin rhythms, and altered clock gene expression have been documented in schizophrenia patients, and these may partly reflect disrupted VIP/VPAC2 signaling in the SCN. Additionally, VIP's anti-inflammatory actions on microglia may be relevant, as neuroinflammation and microglial activation are increasingly recognized as contributing factors in schizophrenia.

NeuroRx Clinical Development Program

NeuroRx, Inc. has been the primary developer of aviptadil as a therapeutic agent. Their clinical program includes:

• IV aviptadil for COVID-19 ARDS (NCT04311697): Phase 2b/3 trial in critically ill patients. Continuous IV infusion at 50–150 pmol/kg/min over 12 hours for 3 consecutive days.
• Inhaled aviptadil (Zyesami) (NCT04536350): Phase 2/3 for non-intubated COVID-19 respiratory failure. 100 µg inhaled via nebulizer three times daily.
• Expanded Access Program: FDA authorized compassionate use for critically ill COVID-19 patients prior to trial completion.
• Regulatory status: NeuroRx filed for Emergency Use Authorization. As of 2024, the FDA pathway remains under review. The clinical landscape has shifted with declining COVID-19 severity and availability of antivirals and updated vaccines.

Pulmonary Arterial Hypertension

Leuchte et al. (2008) conducted a proof-of-concept study demonstrating that inhaled VIP improved pulmonary hemodynamics, exercise capacity, and mixed venous oxygen saturation in patients with primary pulmonary hypertension. The inhaled route achieved therapeutic pulmonary concentrations while minimizing systemic vasodilation and the ultrashort half-life problem associated with IV administration.

Preclinical ARDS Models

Hamidi et al. (2006) demonstrated that VIP administration reduced lung injury severity in animal models of ARDS, decreasing alveolar protein leak, neutrophil infiltration, and pro-inflammatory cytokine levels while improving oxygenation and lung compliance.

Anti-inflammatory Effects in Lung

Beyond COVID-19, aviptadil and VIP have demonstrated anti-inflammatory efficacy in multiple pulmonary disease models. In experimental asthma, VIP reduces airway hyperresponsiveness, mucus hypersecretion, and eosinophilic infiltration. In bleomycin-induced pulmonary fibrosis, VIP attenuates collagen deposition and fibroblast proliferation. In acute lung injury/ARDS models (LPS, acid aspiration), VIP reduces neutrophilic infiltration, edema, and cytokine levels while preserving alveolar epithelial integrity Hamidi et al. (2006).

Sarcoidosis

Prasse et al. (2010) reported that inhaled aviptadil improved lung function and reduced bronchoalveolar lavage TNF-alpha levels in patients with pulmonary sarcoidosis, demonstrating the anti-inflammatory potential of targeted VIP delivery to the lung.

Pulmonary Hypertension

VIP deficiency has been documented in the serum and lung tissue of patients with idiopathic pulmonary arterial hypertension (IPAH). Petkov et al. demonstrated that VIP serum levels were significantly decreased in IPAH patients and that inhaled VIP improved pulmonary hemodynamics and exercise capacity in a small clinical trial Petkov et al. (2003). The synthetic VIP analog aviptadil has also been investigated in critical COVID-19 pneumonia for its lung-protective effects.

Neuroprotection

VIP and its analogs promote neuronal survival through multiple pathways. VIP stimulates release of activity-dependent neuroprotective protein (ADNP) from astrocytes, protects against beta-amyloid toxicity, reduces neuroinflammation, and maintains blood-brain barrier integrity. VIP analogs with extended half-lives — including stearyl-VIP, [Ac-His¹,D-Phe²]-VIP, and lipid-conjugated variants — have shown neuroprotective efficacy in models of Alzheimer's disease, Parkinson's disease, and traumatic brain injury Gozes et al. (2003). ADNP-derived peptides (NAP/davunetide) represent a downstream therapeutic strategy based on VIP's neuroprotective signaling.

Neurological/Immunological Mechanisms

Receptor signaling (VPAC1 and VPAC2):

• VPAC1 (widely expressed: lymphocytes, macrophages, brain, liver, lung): Coupled to Gs → adenylate cyclase → cAMP ↑ → PKA activation → CREB phosphorylation → anti-inflammatory gene transcription
• VPAC2 (smooth muscle, CNS, immune cells): Similar Gs coupling with additional specificity for smooth muscle relaxation and circadian rhythm regulation
• Both receptors also couple to Gq in some cell types → PLC → IP3/DAG → Ca²⁺ signaling
• PAC1 receptor (shared with PACAP): VIP has lower affinity than PACAP for PAC1 but can activate it at higher concentrations

Anti-inflammatory cascade (detailed):

• VPAC1/2 → cAMP ↑ → PKA → phosphorylation and inhibition of IKK complex → blocks NF-κB nuclear translocation
• Suppresses TNF-α, IL-6, IL-12, iNOS, and CXCL8 production by activated macrophages (Delgado et al., PMID: 15082829)
• Induces IL-10 (anti-inflammatory) production through cAMP/PKA-mediated transcription
• Promotes M2 macrophage polarization (anti-inflammatory/tissue repair phenotype)
• Generates tolerogenic dendritic cells (DCs) that express IDO and produce IL-10, promoting Treg induction (Gonzalez-Rey et al., PMID: 17462920)

Immune polarization:

• Shifts T-helper balance from Th1/Th17 (pro-inflammatory, autoimmune) → Th2 (anti-inflammatory)
• Promotes Foxp3+ regulatory T cell generation from naive CD4+ T cells
• Suppresses Th17 differentiation by inhibiting IL-23 receptor expression and RORγt transcription
• This polarization is relevant for autoimmune conditions (RA, MS, IBD) and CIRS

Neuroprotective mechanisms:

• Stimulates release of activity-dependent neuroprotective protein (ADNP) from astrocytes (Gozes & Brenneman, PMID: 10967235)
• ADNP-derived peptide NAP (davunetide) protects against Aβ toxicity, tau hyperphosphorylation, and oxidative stress
• VIP promotes neuronal survival through cAMP/PKA-mediated anti-apoptotic signaling
• Protects against glutamate excitotoxicity by modulating NMDA receptor activity
• Maintains BBB integrity through effects on endothelial tight junction proteins

Ongoing & Future Research

• Aviptadil (synthetic VIP analog) Phase 2/3 trials for COVID-19 respiratory failure (NCT04311697) — results showed improved oxygenation and survival trends
• Development of protease-resistant VIP analogs with extended half-life for more practical dosing
• Research into VIP nanoparticle formulations for targeted delivery to inflamed tissues
• Investigation of VPAC receptor-selective agonists to separate anti-inflammatory effects from unwanted vasodilation
• VIP in neurodegenerative disease: research on VIP analogs (e.g., stearyl-norleucine-VIP) for Alzheimer's disease neuroprotection
• Emerging interest in VIP for post-COVID chronic inflammation and autonomic dysfunction
• Investigation of VIP in inflammatory bowel disease clinical trials (Pozo et al., DOI: 10.3389/fimmu.2022.877505)

Autism Spectrum Disorder

Prenatal VIP levels have been associated with autism risk. Nelson et al. (2001) reported that neonatal blood spot VIP levels were significantly elevated in children later diagnosed with autism compared to typically developing controls Nelson et al. (2001). This finding suggests altered VIP signaling during critical neurodevelopmental windows.

VIP's role in autism may relate to its neurodevelopmental functions — VIP promotes neuronal proliferation, migration, and synaptogenesis through ADNP release, and disruption of VIP signaling during embryonic development produces brain and behavioral abnormalities in animal models. Mice exposed to a VIP antagonist during embryonic days 8-11 develop cortical thinning, reduced hippocampal volume, and behavioral deficits including social interaction impairments and repetitive behaviors. Furthermore, ADNP (activity-dependent neuroprotective protein), which is released by VIP-stimulated astrocytes, is one of the most frequently mutated genes in autism (ADNP syndrome, also called Helsmoortel-Van der Aa syndrome).

CIRS and Mold Illness

VIP has been identified as a critical biomarker in Chronic Inflammatory Response Syndrome (CIRS). Patients with CIRS typically exhibit markedly reduced VIP levels, which correlate with symptoms including fatigue, cognitive dysfunction, shortness of breath, and chronic pain. In the Shoemaker Protocol, intranasal VIP is administered as the final therapeutic step after other markers (MMP-9, TGF-β1, C4a) have been normalized. A prospective trial by Shoemaker et al. demonstrated that intranasal VIP (50 mcg four times daily) significantly improved pulmonary artery systolic pressure, quality of life scores, and inflammatory markers in CIRS patients Shoemaker & House (2006).

Intracavernosal Injection Therapy

Ottesen et al. (1984) first demonstrated that intracavernosal injection of VIP (60 pmol) induced penile tumescence in healthy volunteers, establishing VIP as a physiologically relevant erectogenic peptide. Subsequent studies showed that VIP alone produced tumescence but not full rigidity in most patients with ED, as VIP does not inhibit sympathetic detumescence signaling. This limitation was addressed by combining VIP with the alpha-blocker phentolamine.

COVID-19 ARDS Clinical Trials

Youssef et al. (2022) published results from the Phase II/III trial of intravenous aviptadil in 196 critically ill COVID-19 patients with respiratory failure. The study reported improvements in oxygenation (P/F ratio) and a trend toward reduced mortality in aviptadil-treated patients, though the primary endpoint results were debated. NeuroRx reported that aviptadil demonstrated a statistically significant improvement in survival and recovery at 60 days in patients treated within the first 48 hours of respiratory failure.

Javitt et al. (2021) reported interim data from the Phase II trial showing rapid clinical improvement in 9 of 21 critically ill COVID-19 patients treated with IV aviptadil, with recovery from respiratory failure in patients who were refractory to remdesivir and high-dose corticosteroids.

VIP + Phentolamine Combination (Invicorp)

Dinsmore et al. (1999) conducted a randomized, double-blind, placebo-controlled Phase III trial of Invicorp (VIP 25 mcg + phentolamine 1 mg) in 636 men with ED. The combination produced clinically significant erections in 67% of patients versus 19% for placebo (p<0.001). The response rate was consistent across ED etiologies including vasculogenic, neurogenic, and psychogenic. Invicorp demonstrated a significantly lower incidence of prolonged erection/priapism compared to alprostadil (PGE1) intracavernosal injection.

Anti-inflammatory and Autoimmune Research

VIP has shown therapeutic potential across multiple autoimmune models. In collagen-induced arthritis (a model for rheumatoid arthritis), VIP administration significantly reduced joint inflammation, cartilage destruction, and pro-inflammatory cytokine levels Delgado et al. (2001). In experimental autoimmune encephalomyelitis (EAE, a model for multiple sclerosis), VIP treatment reduced disease severity and demyelination Gonzalez-Rey et al. (2006). These effects are mediated through suppression of Th1/Th17 responses and induction of tolerogenic dendritic cells.

Neuroprotection Against Excitotoxicity

VIP provides neuroprotection against glutamate excitotoxicity in cortical and hippocampal neurons through both direct (VPAC1/cAMP/PKA/CREB) and indirect (astrocyte-mediated ADNP release) mechanisms. Bhave & Bhave (2020) reviewed VIP's neuroprotective effects in models of cerebral ischemia, demonstrating reduced infarct volume and improved neurological outcomes with VIP treatment in MCAO models Bhave & Bhave (2020).

VIP also protects against neuroinflammation-mediated neurodegeneration by shifting microglial phenotype from neurotoxic to neuroprotective states. In models of HIV-associated neurocognitive disorder, Parkinson's disease (MPTP model), and multiple sclerosis (EAE), VIP administration reduces microglial activation, decreases pro-inflammatory cytokine production, and attenuates neuronal loss Delgado & Ganea (2003).

Gastrointestinal Regulation

As a major enteric neuropeptide, VIP regulates intestinal secretion, motility, and mucosal immune homeostasis. It is a key mediator of the "secretory reflex" in the gut and helps maintain immune tolerance to commensal bacteria. Dysregulation of VIP signaling has been implicated in inflammatory bowel disease (IBD), and VIP administration has shown protective effects in experimental colitis models Abad et al. (2003).