ψ-Bufarenogin, a lead compound of anti-cancer drug

Liver cancer is the sixth most common cancer and the second leading cause of cancer death in men at present. There are approximately 700,000 deaths annually worldwide, nearly half of which occurred in China.¹ Surgical resection is only effective in a minority of patients who were diagnosed at the early stage. Resistance of liver cancer to existing conventional chemotherapeutic agents largely contributes to the poor prognosis of patients. Sorafenib is the only effective targeted drug in clinic to date, but its therapeutic effect is rather disappointing. Therefore, the discovery of novel anti-HCC drug remains in an urgent need.

Natural product is an important resource for the active compounds in the drug development. With enormous structural diversity and great biological activity, natural product is hardly replaced by chemical synthesis despite the advances of modern chemistry.² More than half of currently available drugs are natural compounds or their derivatives, and the proportion surpasses 60% in terms of anti-cancer drugs. A good example is Taxol, which was isolated from the Pacific yew tree and was once hailed as the most important anti-cancer drug in decades. Traditional Chinese medicine (TCM), a safe and effective drug broadly used in China for many decades, is regarded as a valuable resource for the lead compound discovery. Among which, the toad skin has been widely applied in liver cancer treatment since the Ming dynasty, and the extract of toad skin is still widely used as a TCM to treat advanced liver cancer in our and other hospitals to date.³ Nevertheless, the effective constituent of toad skin extract remains obscure and the therapeutic effect lacks of scientific explanation, which hinders the recognition and popularization of toad skin extract in American and European countries. In addition, severe adverse effects raised by non-effective components also restrict the application of toad skin extract to a large extent. Therefore, the purification and characterization of novel active compounds with the fewest side effects from toad skin are necessary.

The isolation of active compound from natural product is usually a time-consuming process. We have recently developed a 2-dimensional reversed-phase liquid chromatography/hydrophilic interaction chromatography system with a Click-CD stationary phase, which possessed excellent orthogonality and was successfully utilized to separate the high and intermediate polarity components in toad skin extract.⁴ Through bioassay-guided stepwise isolation, a set of small-molecule compounds including ψ-Bufarenogin, Bufarenogin and Gamabufotalin, etc., were obtained,⁴ among which ψ-Bufarenogin exhibited satisfactory therapeutic effect in xenografted liver cancer without notable side effects (Fig. 1) .

Figure 1.

The flow diagram of ψ-Bufarenogin purification through bioassay-guided stepwise isolation using reversed-phase liquid chromatography coupled with hydrophilic interaction chromatography, and the schematic representation of molecular mechanism underlying the suppressive role of ψ-Bufarenogin on liver cancer progression.

Unlimited proliferation and resistance to apoptosis are 2 distinct hallmarks of cancer cells, and inhibiting proliferation and inducing apoptosis are 2 principle machineries of anti-cancer drugs. ψ-Bufarenogin repressed the proliferation of liver cancer cells via impeding cell cycle transition, and facilitated the cell apoptosis through downregulating Mcl-1 expression. More and more scientists have accepted the concept that cancers are initiated and maintained by cancer stem cells (CSCs), which are able to self-renew and are responsible for chemo-resistance and cancer recurrence.⁵ We found that ψ-Bufarenogin could reduce the liver CSCs expansion through downregulating Sox2 expression, and exhibited synergistic effect with conventional chemotherapeutic agents. Mechanistic study demonstrated that ψ-Bufarenogin dramatically inhibited the activation of MEK/ERK signaling, which is essential in cancer cell proliferation. ψ-Bufarenogin also suppressed PI3-K/Akt pathway, which was required in ψ-Bufarenogin-mediated reduction of Mcl-1 and Sox2. Considering the inhibitory role of ψ-Bufarenogin on both MAPKs and PI3-K/Akt signaling, we speculated that ψ-Bufarenogin might influence the activation of receptor tyrosine kinase (RTK). Epithelial growth factor receptor (EGFR) is regarded as the most important RTK in numerous cancer types, and the significance of EGFR in cancer progression has been verified by the clinical success of its targeted drugs including gefitinib and erlotinib etc.⁶ The hyperactivation of hepatocyte growth factor receptor (c-Met), another RTK, is frequently detected in liver cancers and correlates with the poor prognosis of patients. c-Met usually plays a synergistic role with EGFR and frequently compensates for EGFR activity conferring resistance to EGFR-targeted drugs in cancer.⁷ Unfortunately, there are few targeted drugs in clinic that inhibit EGFR and c-Met simultaneously. Our data showed that ψ-Bufarenogin robustly inhibited the phorylation and activation of both EGFR and c-Met, thereafter suppressed their primary downstream cascades Raf/MEK/ERK and PI3-K/Akt signaling (Fig. 1). These result suggested that ψ-Bufarenogin is a promising lead compound for anti-cancer drug and is worthy to be anticipated in the personalized therapy of the liver cancers with hyperactivated EGFR/c-Met. More importantly, purification and characterization of ψ-Bufarenogin from toad skin sets another impressive example for active compound discovery in nature products.