Inhibition of FABP5 has Anticancer Effect in Treatment-Resistant Prostate Cancer

Prostate Cancer and Castration-Resistant Prostate Cancer

One of the most prevalent cancers in men is prostate cancer (PCa). World Health Organization (WHO) reported that prostate cancer is one of the most common cancers in 2020 with up to 1.41 million cases. Most of the patients have local or regional PCa at diagnosis. However, metastasized prostate cancer has a survival rate of ~30%. Current treatments for metastasized prostate cancer include hormone therapy utilized for reducing the male hormone levels to suppress the growth of prostate cancer cells. In some cases, prostate cancer cells become resistant to hormone therapy, and this type of cancer is known as castration-resistant prostate cancer (CRPC). The average survival time for patients with CRPC is two to three years, and over 90% of patients develop bone metastases.

FABP5 and Retinoic Acid Signaling Pathway

 FABP5 serves as one of the main components for the lipid signaling pathway that causes PCa. In 2019, Carbonetti et al. reported that the activities of fatty acid synthase (FASN) and monoacylglycerol lipase (MAGL), two enzymes that also participate in the lipid signaling pathway as cellular fatty acid pool generators, are heavily dependent on FABP5 expression.37 This finding suggested that FABP5 inhibition may be able to provide a new therapeutic option for mCRPC by modulating lipid signaling.In this lipid signaling pathway, retinoic acid is transported by two distinct chaperons to two separate receptors. Whereas cellular RA-binding protein II (CRABP-II) transports retinoic acids to retinoic acid receptors (RARs), FABP5 transports retinoic acids to peroxisome proliferator activated receptors (PPARs).RAR activation results in the upregulation of genes that facilitate cell cycle arrest and apoptosis. On the other hand, cell proliferation is promoted by activating PPARs. Therefore, to preserve the integrity of the cell cycles, balanced RAR and PPAR activation is essential. In mammary carcinomas, high ratio of the CRABP-II/FABP5 inhibits tumor growth; however, it has been reported that overexpression of FABP5, which lowers the CRABP-II/FABP5 ratio, facilitates carcinoma proliferation.

Figure 1. Fatty acid binding protein 5 serves as a chaperone of FAs from monoacylglycerol lipase (MAGL) and fatty acid synthase (FASN) into nucleus. Activation of peroxisome proliferator-activated receptor and its neighboring genes vascular endothelial growth factor (VEGF) and matrix metallopeptidase (MMP) leads to metastasis. 

FABP5 Inhibitors and Anti-Cancer Effect

In 2017, Al-Jameel et al. reported SBFI-26 to be an inhibitor of the FABP5-PPAR signaling pathway. The MTT assay results indicated cytotoxicity of SBFI-26 and its analog compounds towards PC3-M cell line, while the murine in vivo studies also showed a consistent result in the suppression of PC3-M cells; there was a reduced volume of orthotopically implanted PC3-M cells in the prostate gland of the mouse.Additionally, in 2020, in collaboration with the Kaczocha group, the Ojima research group have shown that SBFI-102 and SBFI-103 not only showed a cytotoxicity towards PCa cells, but also showed a higher effect in reducing tumor growth in a combination with docetaxel and cabezitaxel in vivo. The study suggested that FABP5 inhibitors showed synergetic effect with presently commercialized chemotherapy drugs with less side effects.

Figure 2. (A) 2 SBFI-26 analogs identified to have synergetic effect with docetaxel and cabazitaxel, current available chemotherapy medication on the market, in cytotoxicity towards PCa cells.

Obesity-induced hepatocellular carcinoma

 Addition to prostate cancer cells, FABP5 is upregulated in obesity-induced hepatocellular carcinoma (HCC). Suppression of FABP5 in HCC cells was found to sensitize HCC cells to lipid peroxidation, endoplasmic reticulum stress, and ferroptosis which is a form of cell death dependent on iron and characterized by the accumulation of lipid peroxides. In this study, SBFI-103 was employed as FABP5 inhibitor. Treatment with SBFI-103 enhanced fatty acid oxidation. This induced lipid peroxidation which led to ferroptosis in cancer cells. Treatment of SBFI-103 significantly reduced the abundance of cells that overexpress FABP5 in the tumor, indicating specific ablation of FABP5 overexpressing cancer cells.

 Additionally, inhibition of FABP5 with SBFI-103 showed the accumulation of pro-inflammatory macrophages and cytotoxic T cells. This indicated that treatment of FABP5 inhibitors may enhance anti-tumor immune response in the tumor, which also motivates the immunogenic cell death.

Chemical synthesis of Truxillic acid monoester (TAME) compounds

The synthesis of TAME compounds begins with trans-cinnamic acid or trans-2-methoxycinnamic acid; a [2+2] solid state photoreaction affords the desired α-truxillic acid (α-TA) core in quantitative yields. Then, esterification of α-TA with dehydrating agents such as EDC,HCl affords the α-TAME, but in low yields. To afford γ-TAMEs, α-TA is refluxed in acetic anhydride to the γ-truxillic anhydride intermediate, subsequent nucleophilic attack of the anhydride affords the γ-TAME in moderate to high yields. The ε-TA core is achieved via potassium hydroxide fusion at 350-380°C, then ε-TAMEs are synthesized in similar fashion to α-TAMEs. Previous SAR studies focused on optimizing TAME compounds have modified the ester linkage of the molecule. After eclectically screening in sillico which TAMEs met the desired criteria (docking score, CLogP <7.0, ADMETox filter), TAMEs were then synthesized by Ojima group members. Most of the TAMEs in our work feature a 1,2-biphenyl or 1,3-biphenyl scaffold for the ester linkage; these phenols were afforded via Suzuki–Miyaura coupling in moderate to high yields. To address low yields in the synthesis of α-TAMEs and ε-TAMEs, tert-butyldiphenylsilyl (TBDPS) and 2-(trimethylsilyl)ethanol (TSE) protecting groups were utilized. 

Figure 3. Classical synthetic pathway to afford alpha, gamma, and epsilon TAMEs

TBDPS protection of α-TA affords the desired α-truxillic acid monosilyl ester (α-TAMSE), in addition to unreacted α-TA and the α-truxillic acid disilyl ester (α-TADSE) by product. Using a 2:1 molar ratio of TA:TBDPS-Cl, the α-TAMSE can be isolated in up to 75% yield. Excess α-TA can be recycled through filtration and the α-TADSE can be recycled back into α-TA after acid hydrolysis. With this, 100% of the material can be utilized or recovered, allowing the synthesis of α-TAMEs to be carried out in higher yields, with easier purification, and with greater sustainability.

Figure 4. Sustainable synthesis of the desired TBDPS-TAMSE, followed by TCFH/NMI coupling to afford the desired a-TAME.