Minor withanolides of Physalis longifolia: structure and cytotoxicity

In our recent publication on bioactive guided isolation of compounds from Physalis longifolia (Solanaceae) novel anti-proliferative agents withalongolides A (4) and B (5), and their highly cytotoxic analogues, withalongolide A 4,19,27-triacetate (4a) and withalongolide B 4,19-diacetate (5a) were elucidated. In this study, the two lead compounds (4, 5) were re-isolated in gram quantities for the purpose of further analogue preparation and in vivo testing that would continue to probe structure-activity relationships. During this process, two additional withanolides, named withalongolides O (1) and P (2), were elucidated. Their structures were determined by spectroscopic techniques with 1 being subsequently confirmed by X-ray crystallographic analysis. Utilizing a MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium] viability assay, withalongolide O (1) and its 4,7-diaceatate (1a), both containing the functionalities of Δ(2)-1-oxo- in A ring, a 5β,6β-epoxy in B ring, and a lactone ring in the nine-carbon side chain, exhibited potent cytotoxicity against human head and neck squamous cell carcinoma (JMAR and MDA-1986), melanoma (B16F10 and SKMEL-28), and normal fetal lung fibroblast (MRC-5) cells with IC(50) values in the range between 0.15 and 2.95 µM. In addition, the previously reported α orientation of 7-acetate group in acnistins C and D should be revised to the β orientation on the basis of NMR data comparison.

Novel withanolides target medullary thyroid cancer through inhibition of both RET phosphorylation and the mammalian target of rapamycin pathway

BACKGROUND

Despite development of current targeted therapies for medullary thyroid cancer (MTC), long-term survival remains unchanged. Recently isolated novel withanolide compounds from Solanaceae physalis are highly potent against MTCs. We hypothesize that these withanolides uniquely inhibit RET phosphorylation and the mammalian target of rapamycin (mTOR) pathway in MTC cells as a mechanism of antiproliferation and apoptosis.

METHODS

MTC cells were treated with novel withanolides and MTC-targeted drugs. In vitro studies assessed cell viability and proliferation (MTS; trypan blue assays), apoptosis (flow cytometry with Annexin V/PI staining; confirmed by Western blot analysis), long-term cytotoxic effects (clonogenic assay), and suppression of key regulatory proteins such as RET, Akt, and mTOR (by Western blot analysis).

RESULTS

The novel withanolides potently reduced MTC cell viability (half maximal inhibitory concentration [IC(50)], 270-2,850 nmol/L; 250-1,380 nmol/L for vandetanib; 360-1,640 nmol/L for cabozantinib) with induction of apoptosis at <1,000 nmol/L of drug. Unique from other targeted therapies, withanolides suppressed RET and Akt phosphorylation and protein expression (in a concentration- and time-dependent manner) as well as mTOR activity and translational activity of 4E-BP1 and protein synthesis mediated by p70S6kinase activation at IC(50) concentrations.

CONCLUSION

Novel withanolides from Physalis selectively and potently inhibit MTC cells in vitro. Unlike other MTC-targeted therapies, these compounds uniquely inhibit both RET kinase activity and the Akt/mTOR prosurvival pathway. Further translational studies are warranted to evaluate their clinical potential.

Cytotoxic withanolide constituents of Physalis longifolia

Fourteen new withanolides, 1-14, named withalongolides A-N, respectively, were isolated from the aerial parts of Physalis longifolia together with eight known compounds (15-22). The structures of compounds 1-14 were elucidated through spectroscopic techniques and chemical methods. In addition, the structures of withanolides 1, 2, 3, and 6 were confirmed by X-ray crystallographic analysis. Using a MTS viability assay, eight withanolides (1, 2, 3, 7, 8, 15, 16, and 19) and four acetylated derivatives (1a, 1b, 2a, and 2b) showed potent cytotoxicity against human head and neck squamous cell carcinoma (JMAR and MDA-1986), melanoma (B16F10 and SKMEL-28), and normal fetal fibroblast (MRC-5) cells with IC₅₀ values in the range between 0.067 and 9.3 μM.