Isosilybin

Pharmacology & Research

Isosilybin is a minor flavonolignan congener of the silymarin complex from milk thistle seed — one of the siblings of the dominant silybin (silibinin), alongside silychristin and silydianin. It occurs as two diastereomers, isosilybin A and B, at roughly 10–15% of the mixture, and was among the minor congeners characterised when the flavonolignans were isolated at scale ^{12Reference 12Sy-Cordero A et al. · 2010Large-scale isolation of flavonolignans from Silybum marianum affords new minor constituents and preliminary structure-activity relationshipsView study →}. The load-bearing discipline: milk thistle’s human/clinical evidence (hepatoprotective, NAFLD, the IV Amanita antidote) rests on the silymarin complex or silibinin, not on isolated isosilybin — whose own literature is a coherent but entirely preclinical prostate-cancer/androgen-receptor story, largely from one research group.

1. Anticancer (prostate)

Isosilybin’s distinctive hook. Isosilybin A and B inhibit growth and induce G1 arrest and apoptosis in prostate cancer LNCaP and 22Rv1 cells ^{1Reference 1Deep G et al. · 2007Isosilybin B and isosilybin A inhibit growth, induce G1 arrest and cause apoptosis in human prostate cancer LNCaP and 22Rv1 cellsView study →}; isosilybin B drives androgen-receptor degradation via a PI3K–Akt–Mdm2 pathway ^{2Reference 2Deep G et al. · 2008Isosilybin B causes androgen-receptor degradation in prostate carcinoma cells via a PI3K-Akt-Mdm2-mediated pathwayView study →}, while isosilybin A induces apoptosis by targeting Akt, NF-κB and AR signalling ^{3Reference 3Deep G et al. · 2010Isosilybin A induces apoptosis in human prostate cancer cells via targeting Akt, NF-κB and androgen-receptor signallingView study →}; pure flavonolignans differ in potency, with the isosilybins standing out on AR/PSA-linked readouts ^{4,5Reference 4Deep G et al. · 2008Differential effects of silymarin constituents on cell growth and cell-cycle regulatory molecules in human prostate cancer cellsView study →Reference 5Davis-Searles PR et al. · 2005Milk thistle and prostate cancer: differential effects of pure flavonolignans on antiproliferative endpoints in human prostate carcinoma cellsView study →}. In vivo, isosilybin inhibited advanced prostate tumour growth in nude mice ^{6Reference 6Deep G et al. · 2008AnimalIsosilibinin inhibits advanced human prostate cancer growth in athymic nude mice: comparison with silymarin and silibininView study →}, and pure flavonolignans showed angiopreventive VEGF/VEGFR efficacy ^{7Reference 7Deep G et al. · 2012Angiopreventive efficacy of pure flavonolignans from milk thistle against prostate cancer: targeting VEGF-VEGFR signallingView study →}.

Gap: entirely preclinical (cell lines plus a single mouse xenograft), largely one research group, with AR/PSA suppression a mechanistic readout rather than a clinical outcome and no isolate human trials ^{2,6Reference 2Deep G et al. · 2008Isosilybin B causes androgen-receptor degradation in prostate carcinoma cells via a PI3K-Akt-Mdm2-mediated pathwayView study →Reference 6Deep G et al. · 2008AnimalIsosilibinin inhibits advanced human prostate cancer growth in athymic nude mice: comparison with silymarin and silibininView study →}.

2. Metabolic / anti-lipogenic

Isosilybin regulated lipogenesis and fatty-acid oxidation via the AMPK/SREBP-1c/PPARα axis — mechanistically adjacent to the silymarin-complex NAFLD signal, but shown here for the isolate ^{8Reference 8Liu X et al. · 2022Isosilybin regulates lipogenesis and fatty-acid oxidation via the AMPK/SREBP-1c/PPARα pathwayView study →}.

Gap: a single mechanistic study with no confirmation, no in-vivo metabolic outcome and no human data ^{8Reference 8Liu X et al. · 2022Isosilybin regulates lipogenesis and fatty-acid oxidation via the AMPK/SREBP-1c/PPARα pathwayView study →}.

3. Antioxidant / hepatoprotective

Isosilybin plausibly contributes to the silymarin complex’s antioxidant/hepatoprotective activity as a shared flavonolignan-class property ^{13,14Reference 13Selc M · 2025Looking beyond silybin: the importance of other silymarin flavonolignansView study →Reference 14Agarwal R et al. · 2006Anticancer potential of silymarin: from bench to bedsideView study →}.

Gap: there is no isosilybin-specific hepatoprotection data — this is an inference from complex-level and class-level activity, and milk thistle’s liver trials must not be read as isosilybin evidence ^{14Reference 14Agarwal R et al. · 2006Anticancer potential of silymarin: from bench to bedsideView study →}.

Mechanisms

Target / pathway	Effect	Relevant to
Androgen receptor (AR)	isosilybin B → AR degradation via PI3K–Akt–Mdm2; ↓ AR/PSA	prostate anticancer
Akt / NF-κB	isosilybin A → pro-apoptotic, inhibits survival signalling	prostate anticancer
Cell cycle (G1)	arrest, growth inhibition	prostate anticancer
VEGF–VEGFR	angiopreventive	prostate anticancer
AMPK / SREBP-1c / PPARα	↓ lipogenesis, ↑ fatty-acid oxidation	metabolic
CYP2C9	isosilybin B = potent inhibitor (clinically achievable concentrations)	drug interactions / safety
PXR → CYP3A4	silybin/isosilybin inhibit PXR-mediated CYP3A4 induction	drug interactions / safety

Diastereomer note: isosilybin B is repeatedly the more active/selective congener at AR degradation, CYP2C9 and PXR-linked targets.

Pharmacokinetics

There is no dedicated human pharmacokinetic study of isolated isosilybin. As a milk-thistle flavonolignan it shares silibinin’s PK class — poor oral bioavailability (low aqueous solubility, extensive phase-II conjugation, rapid clearance), so teas/infusions do not deliver meaningful systemic levels ^{13Reference 13Selc M · 2025Looking beyond silybin: the importance of other silymarin flavonolignansView study →}. At the component level, silymarin flavonolignans and their sulfate metabolites bind human serum albumin and interact with CYP2C9/2C19/2D6/3A4 ^{11Reference 11Faisal Z et al. · 2021Interaction of silymarin components and their sulfate metabolites with human serum albumin and CYP (2C9, 2C19, 2D6, 3A4) enzymesView study →}, and isosilybin B behaves as a more potent ligand/inhibitor at CYP2C9 ^{9Reference 9Brantley SJ et al. · 2010Two flavonolignans from milk thistle inhibit CYP2C9-mediated warfarin metabolism at clinically achievable concentrationsView study →} and PXR-linked CYP3A4 induction ^{10Reference 10Mooiman KD et al. · 2013Milk thistle’s active components silybin and isosilybin: novel inhibitors of PXR-mediated CYP3A4 inductionView study →} than several siblings — relevant to interaction risk rather than to therapeutic exposure.

Clinical trials

There are no human trials of isolated isosilybin (A or B) as a single agent. All milk-thistle clinical evidence (hepatoprotective/NAFLD, antidiabetic, the IV Amanita antidote) is for the silymarin complex or silibinin — this congener’s own profile is strictly preclinical.

Last checked: July 2026.

Toxicity & Safety

Isosilybin has a low toxicity signal — no isolate-specific toxicity figures exist, and as a minor component of silymarin (well tolerated in humans) it carries no distinct toxicity signal. The honest caveat is an interaction one, inherited and sharpened from the class: silymarin flavonolignans inhibit CYP2C9 and CYP3A4 (including PXR-mediated induction), and isosilybin B is specifically a potent CYP2C9 inhibitor at clinically achievable concentrations — relevant to warfarin and other narrow-therapeutic-window CYP2C9 substrates ^{9,10,11Reference 9Brantley SJ et al. · 2010Two flavonolignans from milk thistle inhibit CYP2C9-mediated warfarin metabolism at clinically achievable concentrationsView study →Reference 10Mooiman KD et al. · 2013Milk thistle’s active components silybin and isosilybin: novel inhibitors of PXR-mediated CYP3A4 inductionView study →Reference 11Faisal Z et al. · 2021Interaction of silymarin components and their sulfate metabolites with human serum albumin and CYP (2C9, 2C19, 2D6, 3A4) enzymesView study →}. Asteraceae-allergy caution applies to milk-thistle products generally.

Pregnancy & lactation

Avoid the isolate. No reproductive or developmental data exist for isosilybin, and milk thistle itself is not formally studied in pregnancy; any lactation/galactagogue tradition attaches to the whole herb, not this isolated congener, and is not a safety clearance.

Dosage

There is no established dose — isosilybin is not sold or dosed as an isolate and is consumed only as ~10–15% of standardised silymarin extracts, so no monotherapy dosing exists and none should be implied. Silymarin-complex dosing belongs on the silymarin/milk-thistle pages.

References