Abstract: Apoptosis and mitochondrial dysfunction are central to various pathological processes, including neurodegenerative and cardiovascular diseases characterized by myocyte and tissue degeneration. The voltage-dependent anion channel 1 (VDAC1), located at the outer mitochondrial membrane, is a critical convergence point for cell survival and death signals. Recent high-throughput screening and medicinal chemistry efforts have identified VBIT-4, a novel small molecule that directly targets VDAC1. VBIT-4 effectively inhibits VDAC1 oligomerization, thereby preventing the release of apoptogenic proteins like cytochrome c, maintaining mitochondrial membrane potential, and halting the detachment of hexokinase from mitochondria. By preventing these early apoptotic events, VBIT-4 protects cells—including cardiac myocytes and neurons—from apoptosis and mitochondrial dysfunction without exhibiting in vivo toxicity. This review summarizes the pharmacological activity, molecular mechanism, and structure-activity relationship of VBIT-4, highlighting its potential as a therapeutic agent for diseases driven by aberrant apoptosis and muscle/tissue degeneration.
1. Introduction
Mitochondria play a crucial role in cellular energy generation, metabolism, calcium homeostasis, and cell death. Mitochondrial dysfunction is a hallmark of numerous human diseases, including neurodegenerative disorders and cardiovascular diseases such as heart failure, myocardial infarction, and ischemia/reperfusion injury, which involve the apoptosis, necrosis, and autophagy of cardiac myocytes [1]. A key player in mitochondrion-mediated apoptosis is the voltage-dependent anion channel 1 (VDAC1), a protein located at the outer mitochondrial membrane (OMM) that serves as a mitochondrial gatekeeper [1].
During apoptosis, VDAC1 undergoes oligomerization to form a large flexible pore that allows the release of pro-apoptotic proteins, such as cytochrome c (Cyto c), from the intermembrane space into the cytosol, triggering the caspase cascade [1]. Because VDAC1 oligomerization is a critical early step in apoptosis, it represents a prime pharmacological target. Through high-throughput bioluminescence resonance energy transfer (BRET2)-based screening of a library of 1,468 drug-like compounds, followed by medicinal chemistry optimization, a novel compound named VBIT-4 was developed [1]. VBIT-4 specifically interacts with VDAC1 to inhibit its oligomerization, offering a promising therapeutic strategy for conditions associated with enhanced apoptosis and myocyte degeneration [1].
2. Pharmacological Activity
VBIT-4 demonstrates potent anti-apoptotic and cytoprotective pharmacological activities across various cell lines. It effectively inhibits apoptosis induced by multiple agents, including selenite, cisplatin, and staurosporine (STS), with an IC50 value in the low micromolar range (approximately 1.8 to 2.9 μM) [1]. The compound's protective effects extend to preserving mitochondrial function during pathological stress.
Specifically, VBIT-4 prevents the dissipation of the mitochondrial membrane potential (ΔΨm) and inhibits the overall cellular and mitochondrial production of reactive oxygen species (ROS) [1]. Furthermore, apoptosis induction typically disrupts cellular calcium homeostasis; VBIT-4 successfully prevents the elevation of intracellular calcium ([Ca2+]i) associated with apoptosis, thereby preventing calcium accumulation in the mitochondria and subsequent metabolic collapse [1]. Importantly, VBIT-4 has no adverse effects on normal cell growth and viability. In vivo studies demonstrated that mice exposed to VBIT-4 for 4 months showed no signs of toxicity, as reflected by weight, behavior, and organ histochemistry, indicating that its interaction with VDAC isoforms does not disrupt functional cellular homeostasis [1].
3. Molecular Mechanism of Action
The primary mechanism of action of VBIT-4 involves its direct interaction with VDAC1, which prevents the protein from shifting from a monomeric to an oligomeric state [1]. By inhibiting VDAC1 oligomerization, VBIT-4 blocks the formation of the large pore required for the transit of Cyto c and other apoptogenic factors across the OMM [1]. Microscale thermophoresis (MST) assays confirm that VBIT-4 binds directly to purified VDAC1 and reduces its channel conductance [1].
Additionally, VBIT-4 plays a crucial role in regulating mitochondrial metabolism by preventing the detachment of hexokinase (HK), specifically HK-I and HK-II, from the mitochondria [1]. Mitochondrion-bound HK couples cytosolic glycolysis to mitochondrial oxidative phosphorylation and prevents the release of pro-apoptotic factors. Apoptotic stimuli typically induce the dissociation of the VDAC1-HK complex; however, VBIT-4 strongly inhibits this detachment, supporting the concept that HK detachment is associated with VDAC1 oligomerization and is a prerequisite for apoptosis [1]. The protection conferred by VBIT-4 is VDAC1-dependent but independent of the pro-apoptotic Bax and Bak proteins, as it remains effective in Bax-/-/Bak-/- cells [1].
4. Structure-Activity Relationship (SAR)
VBIT-4 was synthesized through the structural optimization of a hit compound, AKOS-022. Two main structural modifications were made to the AKOS-022 core to generate VBIT-4: the piperazine ring was conjugated with an aniline moiety (instead of a piperidine), and the central pyrrolidine-2,5-dione ring was linearized [1].
This linearization resulted in a 4-hydroxybutanamide moiety, which is the key structural difference distinguishing VBIT-4 from its less potent analog, VBIT-3 [1]. This change makes VBIT-4 significantly more flexible (due to the linear moiety versus a cyclic group) and more hydrophilic (alcohol versus amide). The increased flexibility is critical for its ability to interfere with the protein-protein interactions required for VDAC1 oligomerization [1]. Furthermore, VBIT-4 is a chiral molecule containing a chiral carbon. However, SAR studies separating the two enantiomers (VBIT-4-1 and VBIT-4-2) revealed that both R and S conformations exhibit identical activity in inhibiting VDAC1 oligomerization and apoptosis. This suggests that the flexible VBIT-4 molecule interferes with VDAC1 monomer interactions in binding pockets that are widely accessible to both enantiomeric conformations [1].
5. Current Limitations
While VBIT-4 is highly effective against various apoptotic triggers, it exhibits a paradoxical effect when apoptosis is induced by arsenic trioxide (As2O3). Instead of inhibiting cell death, the addition of VBIT-4 slightly enhances As2O3-induced apoptosis [1]. This limitation is likely due to As2O3 possessing distinct molecular targets and mechanisms of action that are either unaffected or exacerbated by the inhibition of VDAC1 oligomerization, such as direct ROS formation, down-regulation of Bcl-2 expression, or direct binding to promyelocytic leukemia proteins [1].
Additionally, while VBIT-4 binds to native VDAC1 purified from rat liver mitochondria with high affinity, it interacts with recombinant VDAC isoforms (VDAC1, VDAC2, and VDAC3) with a roughly 3-fold lower affinity (approximately 53 μM) [1]. This discrepancy may result from the unfolding and refolding processes used to prepare the recombinant proteins, which can lead to modified conformations that do not perfectly mimic the native protein structure in the mitochondrial membrane [1].
6. Future Perspectives
The development of VBIT-4 provides a powerful pharmacological tool to further explore the physiological and pathological roles of VDAC1 in controlling metabolism, energy production, and apoptosis [1]. Because VDAC1 overexpression and subsequent oligomerization are implicated in the apoptosis observed in cardiovascular diseases (such as atrial fibrillation and heart failure) and neurodegenerative disorders, VBIT-4 represents a highly attractive therapeutic candidate [1].
Future research should focus on evaluating VBIT-4 in in vivo models of cardiac myocyte degeneration, ischemia/reperfusion injury, and other muscle-wasting or neurodegenerative conditions linked to mitochondrial dysfunction. By preventing HK detachment, ROS generation, and calcium overload, VBIT-4 and its future derivatives hold significant promise for halting the progression of fibrosis and tissue degeneration in patients suffering from severe apoptosis-associated disorders [1].
7. References
