CA-074 Me: Selective Cathepsin B Inhibitor for Lysosomal Pathway Research

Executive Summary: CA-074 Me is a methyl ester derivative of CA-074, optimized for membrane permeability and potent, selective inhibition of cathepsin B (IC₅₀ = 36.3 nM) [APExBIO]. It blocks intracellular cathepsin B, modulates apoptosis and necroptosis, and attenuates TNF-α-induced liver injury in vivo (Liu et al., 2024). Under reducing conditions, it partially inhibits cathepsin L, demonstrating context-dependent selectivity. CA-074 Me is insoluble in water, but dissolves readily in DMSO (≥19.88 mg/mL) and ethanol (≥51.5 mg/mL with sonication). Its rapid action and well-characterized mechanism support its widespread use in apoptosis, lysosomal membrane permeabilization (LMP), and inflammation research [ca-074.com].

Biological Rationale

Cathepsin B is a lysosomal cysteine protease, essential for protein degradation and implicated in apoptosis, necroptosis, and inflammatory signaling (Liu et al., 2024). Lysosomal membrane permeabilization (LMP) releases cathepsins, including cathepsin B, into the cytosol, where they cleave substrates critical for cell survival. This process is central to cell death cascades triggered by TNF-α, oxidative stress, and certain chemotherapeutics [hypoxanthine.com]. Inhibiting cathepsin B blocks downstream proteolytic events required for apoptosis and necroptosis, providing a tool to dissect lysosomal contributions to regulated cell death [cathepsinsinhibitor.com]. CA-074 Me’s cell permeability and selectivity enable intervention at the lysosomal stage, preventing cathepsin B–mediated cleavage during LMP and related cell death processes.

Mechanism of Action of CA-074 Me (Cathepsin B inhibitor)

CA-074 Me (SKU: A8239) is a methyl ester derivative of CA-074, designed for efficient cell entry and intracellular cathepsin B inhibition [APExBIO]. The compound irreversibly binds the active-site cysteine of cathepsin B, blocking substrate access and subsequent proteolysis. In vitro, CA-074 Me inhibits cathepsin B with an IC₅₀ of 36.3 nM under standard buffer conditions (pH 5.5, 25°C). In the presence of reducing agents (1 mM DTT or GSH), it also partially inhibits cathepsin L (>90% inhibition after pre-incubation), but is otherwise highly selective for cathepsin B (Liu et al., 2024). CA-074 Me’s methyl ester group facilitates plasma membrane permeability. Once inside the cell, intracellular esterases convert it to the active acid form (CA-074), which accumulates in lysosomes and inhibits cathepsin B at the source of LMP-induced cell death.

Evidence & Benchmarks

• CA-074 Me inhibits purified human cathepsin B activity with an IC₅₀ of 36.3 nM (25°C, pH 5.5) (APExBIO).
• Pre-incubation with DTT or GSH enables >90% inhibition of cathepsin L, highlighting the compound’s context-dependent selectivity (Liu et al., 2024).
• In necroptosis models, chemical inhibition or siRNA knockdown of cathepsin B using CA-074 Me reduces cell death after TNF/Smac-mimetic/Z-VAD-FMK stimulation (Liu et al., 2024).
• In mouse models, CA-074 Me attenuates TNF-α-induced liver damage and reduces hepatocyte apoptosis (Liu et al., 2024).
• CA-074 Me is insoluble in water but dissolves in DMSO (≥19.88 mg/mL) and ethanol (≥51.5 mg/mL with sonication), supporting its use in diverse assay formats (APExBIO).
• In cultured cells, CA-074 Me reduces cathepsin B activity and blocks apoptosis induced by bile salts and TNF-α. (cathepsinsinhibitor.com)

Applications, Limits & Misconceptions

CA-074 Me enables mechanistic dissection of the cathepsin B signaling axis in apoptosis, necroptosis, and inflammation research. Its membrane permeability and rapid action make it suitable for live-cell, biochemical, and in vivo models. Common applications include:

• Apoptosis Assays: Use to block lysosomal-mediated apoptosis in response to bile salts, TNF-α, or chemotherapeutics.
• Necroptosis Models: Dissect MLKL polymerization-induced lysosomal membrane permeabilization (LMP) and cathepsin B–dependent cell death (Liu et al., 2024).
• Lysosomal Pathway Studies: Probe lysosome integrity, protease release, and downstream signaling events.
• Inflammatory Liver Disease Models: Attenuate TNF-α-induced liver injury in animal studies (Liu et al., 2024).

This article extends the detailed methodological workflows in Optimizing Lysosomal Protease Inhibition by providing updated benchmarks from peer-reviewed studies and clarifying selectivity under reducing conditions.

Compared to CA-074 Me: Precision Cathepsin B Inhibitor for Lysosomal ..., this article emphasizes experimental boundaries and integrates recent in vivo liver injury models.

Common Pitfalls or Misconceptions

• Not selective for cathepsin B under reducing conditions: CA-074 Me also inhibits cathepsin L when pre-incubated with DTT or GSH. Use non-reducing conditions for strict cathepsin B selectivity (Liu et al., 2024).
• Not water-soluble: The compound is insoluble in water; dissolve in DMSO or ethanol with sonication for stock solutions (APExBIO).
• Short solution stability: Solutions should not be stored long-term; prepare fresh before each experiment (APExBIO).
• Does not inhibit all lysosomal proteases: CA-074 Me is ineffective against cathepsins D, S, and other family members under standard conditions.
• Cannot reverse established cell death: CA-074 Me blocks initiation of cathepsin B-dependent pathways but does not rescue cells once downstream apoptosis or necroptosis is underway.

Workflow Integration & Parameters

For in vitro assays, dissolve CA-074 Me in DMSO to a stock concentration of 10–20 mM. For cell culture, typical working concentrations range from 1–10 μM, with pre-incubation times of 15–60 minutes at 37°C. For animal studies, dosing regimens should be referenced from published protocols (e.g., 10 mg/kg, intraperitoneal, in mouse TNF-α-induced liver injury models) (Liu et al., 2024). Avoid prolonged storage of working solutions, and use immediately after preparation. Confirm cathepsin B inhibition by fluorescence substrate assays or immunoblotting for cleaved substrates. For troubleshooting and advanced assay design, see CA-074 Me: Cell-Permeable Cathepsin B Inhibitor for Lysos..., which this article updates by integrating new findings on MLKL-induced LMP and necroptosis.

Conclusion & Outlook

CA-074 Me, offered by APExBIO, is a validated, cell-permeable, and potent cathepsin B inhibitor that empowers researchers to dissect lysosomal cell death pathways, apoptosis, and inflammation with precision. Its benchmarked selectivity, robust in vitro and in vivo efficacy, and compatibility with advanced cellular models make it indispensable for apoptosis and necroptosis studies. Ongoing research will further define its translational utility in inflammatory liver disease and cancer biology. For product specifications and ordering, visit the CA-074 Me (Cathepsin B inhibitor) product page.