Ipriflavone
1. Identity and Chemical Nature
Systematic and common names: Ipriflavone has the chemical structure 7-isopropoxyisoflavone and is derived from the soy isoflavone daidzein. It is also known by the name ipraflavone and is abbreviated in the scientific literature as IP. Other synonyms include 7-isopropoxy-isoflavone.
Chemical classification: Ipriflavone is a mass-produced synthetic derivative of daidzein. Genistein and daidzein are unique plant compounds called isoflavones, primarily found in soy products. Isoflavones belong to a larger category known as flavonoids, which are natural plant components that have antioxidant, anti-inflammatory, anti-allergy, and anticancer properties.
Physical and molecular properties: Ipriflavone (CAS Number: 35212-22-7, molecular formula: C₁₈H₁₆O₃, molecular weight: 280.32 g/mol) typically appears as a white to off-white crystalline powder with three isopropyl groups substituted on the basic isoflavone structure, produced through controlled chemical synthesis.
Relationship to natural isoflavones: Ipriflavone is a synthetic isoflavone compound structurally derived from naturally occurring daidzein. While daidzein and related isoflavones occur naturally in legumes, ipriflavone itself is synthesized commercially. Although most soy isoflavones are classified as plant estrogens (phytoestrogens), ipriflavone does not have estrogenic activity and does not activate any estrogen receptors in the body.
Available forms: Ipriflavone is made in the laboratory from a compound found in the soy plant. In some countries, ipriflavone is available as a prescription. In the United States, ipriflavone is considered a dietary supplement. It is marketed under trade names including Osteochin in Hungary and Osteofix in certain other markets. A pilot study confirmed an 8- to 20-fold increase in the bioavailability of ipriflavone after administration of tablets in the fed state relative to the fasting state.
2. History and Development
Discovery: Ipriflavone (7-isopropoxyisoflavone) was discovered in 1969 by investigators in Budapest while studying plant growth factors. Ipriflavone was first isolated at a Hungarian pharmaceutical company in 1969. Although the compound was first described in the 1930s in earlier chemical literature, in 1969 a research project was initiated to manufacture a type of isoflavone that would possess the bone-stimulating effects of estrogen without any estrogen-like activity elsewhere in the body. Such a product would help prevent osteoporosis but cause no other health risks. Ipriflavone was the result.
Preclinical and early clinical development: After seven successful years of experiments with animals, human research was started in 1981. Over 150 studies on safety and effectiveness, both animal and human, have been conducted in Italy, Hungary, and Japan. As of 1997, 2,769 patients had been treated a total of 3,132 patient years.
Regulatory approval and international status: Ipriflavone has been available for prevention of osteoporosis since 1989 in over 22 countries. Since the 1980s, it has been a registered prescription drug for the prevention and treatment of osteoporosis in Japan, Argentina, and Europe. It is an accepted treatment for osteoporosis in Italy and Japan. Its efficacy in treating osteoporosis has been recognized by the NMPA of China in 2001. In Hungary, ipriflavone has been marketed under the name Osteochin. The results of clinical experiments in Hungary, Japan, and Italy are similar.
3. Pharmacokinetics and Metabolic Fate
Absorption: Ipriflavone is metabolized mainly in the liver and excreted in the urine. Food appears to enhance its absorption. Food items, particularly those containing lipids, increase the small intestine's absorption of ipriflavone.
First-pass metabolism: Approximately 30% of ipriflavone absorbed into the portal vein was eliminated by the liver (hepatic first-pass effect) based on intravenous and intraportal administration studies. The low bioavailability after oral administration in rats was mainly due to intestinal first-pass effect. The hepatic first-pass effect and incomplete absorption from the gastrointestinal tract also contributed to the low bioavailability.
Metabolites: In dogs and rats, seven metabolites were identified in the plasma. In humans, however, only MI, MII (daidzein), MIII, and MV seem to predominate. Notably, one of ipriflavone's principal human metabolites is daidzein, the same natural soy isoflavone from which ipriflavone is structurally derived. The metabolites, especially M2 and M5, also have activity.
Half-life and excretion: The mean excretion half-life in healthy human volunteers was 9.8 hours for ipriflavone and ranged from 2.7–16.1 hours for its metabolites. Ipriflavone is rapidly metabolized and quickly eliminated.
4. Key Constituents and Mechanisms of Action
4.1 Inhibition of Bone Resorption
Inhibition of bone resorption was demonstrated in several models, both in vitro and in vivo, for IP and its metabolites. Their mechanisms of action on bone are not yet fully elucidated but some of them are widely accepted. IP does not possess, per se, any estrogenic activity. It appears that IP-related inhibition of bone resorption might be mediated by an indirect effect on osteoclasts, related to an inhibition of recruitment and/or differentiation of pre-osteoclasts, possibly through a modulation of osteoblast response to parathyroid hormone (PTH).
The mechanism is thought to involve the inhibition of bone resorption. Ipriflavone inhibited formation of osteoclasts from murine spleen cells co-cultured with stromal cells cloned from murine bone marrow. In this system, ipriflavone inhibited osteoclast generation in a dose-dependent manner (10⁻⁷–10⁻⁵ M). Ipriflavone also inhibited prostaglandin E2 production in MC3T3-E1 cells (widely employed as osteoblasts) and inhibited the proliferation of stromal cells (10⁻⁶–10⁻⁵ M), but not osteoblastic cells. These results suggest that one mechanism for the inhibitory effects of ipriflavone on bone resorption is the inhibition of osteoclast formation through inhibiting prostaglandin E2 production in osteoblasts and thereby suppressing proliferation of stromal cells.
4.2 Novel Ipriflavone Receptors on Osteoclasts
Radioligand binding studies indicated the presence of specific ipriflavone binding sites (two classes) in both precursor cells and in mature osteoclasts. Specific ipriflavone binding was not displaced by various modulators of avian osteoclast function such as estradiol or retinoic acid, indicating that ipriflavone receptors differ from the receptors for these calcium-regulating hormones. Novel specific ipriflavone receptors that are coupled to Ca²⁺ influx were demonstrated in osteoclasts and their precursor cells. These ipriflavone receptors may provide a mechanism to regulate osteoclast differentiation and function.
4.3 Stimulation of Bone Formation (Osteoblastic Effects)
Ipriflavone and metabolites stimulate human osteoblast differentiation and function, enhancing expression of matrix proteins and facilitating the mineralization process. Pretreatment of osteoblasts with ipriflavone demonstrated effects on parathyroid hormone (PTH) response, collagen synthesis, and cell proliferation, again indicating a modulation of synthetic and growth properties of bone.
Ipriflavone and one of its metabolites stimulated cell proliferation of an osteoblast-like cell line (UMR-106a, often used to study the effects of hormones and drugs on bone metabolism). Ipriflavone and one of its metabolites increased alkaline phosphatase activity, while another metabolite stimulated collagen formation. In addition, ipriflavone alone is able to inhibit parathyroid hormone activity.
4.4 Calcitonin-Related Pathway
Preclinical experimental examinations performed in Hungary and Japan found that ipriflavone stimulates calcitonin secretion through increasing the estrogen effect. Its effect of hindering bone resorption works presumably also through this pathway.
4.5 Absence of Direct Estrogenic Activity
Although ipriflavone is non-estrogenic, its exact mechanism for supporting bone development is not fully understood. Ipriflavone is considered a selective estrogen receptor modulator and supports the growth of osteoblasts and regulates the multiplication of osteoclasts. This compound is devoid of estrogenic activity in humans. One of the advantages of ipriflavone in the treatment of osteoporosis is the absence of a direct estrogenic effect. In a study, ipriflavone or placebo was administered to 15 postmenopausal women. LH, FSH, prolactin, and estradiol were measured after a single oral dose of 600 mg or 1000 mg and after 7, 14, and 21 days of treatment. There were no differences in endocrine parameters between the ipriflavone and placebo groups.
5. Body Systems and Health Areas
Ipriflavone's research base is almost entirely focused on the skeletal system, particularly bone metabolism. Ipriflavone is used for treatment of osteoporosis, Paget's disease, and hyperparathyroidism — conditions of high bone turnover. Preliminary studies have also found ipriflavone effective in preventing bone loss associated with chronic steroid use, immobility, ovariectomy, renal osteodystrophy, and gonadotropin hormone-releasing hormone agonists. In addition, it holds promise for the treatment of other metabolic diseases affecting the bones, including Paget's disease of the bone, hyperparathyroidism, and tinnitus caused by otosclerosis.
6. Scientific Evidence by Area of Use
6.1 Postmenopausal Osteoporosis and Osteopenia — Main Evidence Base
Early and smaller clinical trials (largely positive): Research suggests that ipriflavone, when combined with calcium, may slow down the progression of osteoporosis and even lead to modest gains in bone density over time. Ipriflavone is a flavonoid derivative that has been demonstrated to have both direct and indirect effects on the skeleton. Not only does this drug inhibit the differentiation of precursors to osteoblasts and inhibit bone resorption, it also appears to stimulate osteoblast production and activity. A controlled clinical trial in 70 early postmenopausal women demonstrated that ipriflavone 200 mg three times daily orally increased BMD in the distal radius an average of 5% per year during 2 years of therapy.
A double-blind study (n = 48) found that BMD increased by 5.6% in the lumbar spine and 3.6% in the femoral neck in the ipriflavone group.
GnRH-agonist–induced bone loss: In a double-blind, placebo-controlled study, ipriflavone (600 mg/day) or identical placebo tablets were given with 500 mg/day of calcium to patients treated with the GnRH agonist leuproreline acetate (3.75 mg every 30 days for 6 months). In placebo-treated subjects (n = 39), urinary hydroxyproline excretion and plasma osteocalcin levels showed a significant increase, while spine bone density and total body bone density significantly decreased after 3 and 6 months. In the ipriflavone-treated group (n = 39), no significant difference in bone markers and bone density was observed. These data indicate that ipriflavone can restrain the bone remodeling processes and prevent the rapid bone loss that follows medically induced hypogonadism.
The Ipriflavone Multicenter European Fracture Study (IMEFS) — largest and most cited trial: The largest ipriflavone trial to date is the Ipriflavone Multicenter European Fracture Study, a prospective, randomized, double-blind, placebo-controlled study conducted in 4 centers in Belgium, Denmark, and Italy between August 1994 and July 1998. Four hundred seventy-four postmenopausal white women, ages 45 to 75 years, with low BMD were randomly assigned to receive ipriflavone 200 mg (n = 234) or matching placebo (n = 240) three times per day; all received 500 mg/day calcium. The primary outcome measure was reduction in fracture incidence. During the study period, no difference was noted in the occurrence of vertebral fractures among women taking ipriflavone compared with those taking placebo.
Based on intent-to-treat analysis, after 36 months of treatment, the annual percentage change from baseline in BMD of the lumbar spine for ipriflavone vs. placebo (0.1% vs. 0.8%; P = 0.14), or in any of the other sites measured, did not differ significantly between groups. The response in biochemical markers was also similar between groups. The number of women with new vertebral fractures was identical or nearly so in the two groups at all time points.
2020 systematic review and meta-analysis: The systematic review and meta-analysis (PubMed, CENTRAL, CNKI; articles from 1990 through January 2015) concluded that IP significantly increases BMD and has an inhibitory effect on bone resorption markers in postmenopausal women with osteopenia or osteoporosis. Gastrointestinal symptoms may occur, but adverse drug withdrawal events were not statistically increased when compared with the placebo group.
Overall evidence assessment: There continues to be controversy regarding the efficacy and safety due to some contradictory reports. According to all but one study, ipriflavone combined with calcium can slow and perhaps slightly reverse bone breakdown. The IMEFS, the largest and most rigorously designed trial, did not confirm a fracture-reduction benefit or a statistically significant effect on BMD at standard doses over 3 years, representing an important qualification of the earlier, smaller positive studies.
6.2 Paget's Disease of Bone
Ipriflavone, an isoflavone derivative, seems to prevent the loss of bone mass through the inhibition of bone resorption, mainly by inhibiting the recruitment of osteoclasts. An investigation examined whether a brief course of treatment with IP could reduce biochemical parameters of accelerated bone turnover and bone pain in patients with active Paget's disease of bone. Sixteen patients (9 males and 7 females) with active Paget's disease were randomly allocated to two different crossover dose regimens of treatment with IP (600 mg/day vs. 1200 mg/day), each lasting 30 days with a 15-day washout period. Serum alkaline phosphatase and urinary hydroxyproline/creatinine excretion were reduced after each sequence. At the end of the 600/1200 mg/day treatment sequence, serum alkaline phosphatase and hydroxyproline/creatinine decreased by 32% and 25.6%, respectively.
Clinical studies in Paget's disease of bone or primary hyperparathyroidism confirmed preferential inhibition of bone resorption. The evidence here is from small, short-term studies and is considered preliminary.
6.3 Renal Osteodystrophy
Ipriflavone is also used for reducing bone loss caused by chronic kidney disease (renal osteodystrophy). A preliminary clinical report (Hyodo et al., Nephron 1991) examined the effects of ipriflavone administration in hemodialysis patients with renal osteodystrophy. This evidence remains preliminary and confined to small, early-phase studies.
6.4 Drug-Induced Bone Loss (Steroids, GnRH Agonists)
Preliminary studies have found ipriflavone effective in preventing bone loss associated with chronic steroid use, immobility, ovariectomy, renal osteodystrophy, and gonadotropin hormone-releasing hormone agonists. The evidence in these specific populations is based on small, often single-center trials and must be characterized as preliminary.
6.5 Osteoporosis-Related Bone Pain
Ipriflavone appears to alleviate pain associated with fractures caused by osteoporosis. Ipriflavone's advantages include that it decreases acute and chronic back pain due to osteoporosis. A study by Scali et al. (Curr Ther Res 1991) examined the analgesic effect of ipriflavone versus calcitonin in the treatment of osteoporotic vertebral pain. The pain-relief evidence remains limited to small, early trials.
6.6 Immobilization-Related Bone Loss
Ipriflavone is used for preventing and treating weak bones (osteoporosis) in older women and for reducing bone loss caused by paralysis associated with stroke. Researchers have found that paralyzed stroke patients have weaker bones on the affected side, possibly due to immobility as well as vitamin D deficiency. This evidence is again limited to early and small studies.
6.7 Combination with Estrogen Therapy
Ipriflavone is sometimes combined with low-dose estrogen preparations. While ipriflavone appears to enhance estrogen's effect, it does not possess intrinsic estrogenic activity, making it an attractive adjunct or alternative to conventional hormone replacement therapy. Several Italian clinical trials from the 1990s examined ipriflavone in combination with low-dose conjugated equine estrogen in oophorectomized and postmenopausal women, reporting additive effects on bone density preservation, though these trials are small and primarily exploratory.
7. Dosage Forms and Dosages Reported in Studies
Ipriflavone is commercially available primarily in oral form as tablets or capsules. For osteoporosis, 200 mg of ipriflavone three times daily has been studied in scientific research. This yields a total daily dose of 600 mg, which is the most widely studied regimen. For Paget's disease, doses of 600–1200 mg of ipriflavone daily have been studied. For treating weak bones due to kidney disease (renal osteodystrophy), 400–600 mg of ipriflavone daily has been reported in studies.
In the IMEFS, participants received ipriflavone 200 mg three times per day (total 600 mg/day), and all received 500 mg/day calcium supplementation alongside. In the GnRH agonist bone-loss study, 600 mg/day total daily dose was co-administered with 500 mg/day of calcium.
A pilot pharmacokinetic study showed an 8- to 20-fold increase in ipriflavone bioavailability after administration of tablets in the fed state relative to the fasting state, confirming the clinical importance of administering ipriflavone with food.
8. Safety Considerations and Drug Interactions
8.1 Lymphocytopenia
The most clinically significant safety signal identified in ipriflavone research is a reduction in circulating lymphocytes. A multicenter study showed that around 13% of postmenopausal women who daily received 600 mg ipriflavone for 3 years developed lymphocytopenia (lymphocyte counts <500 × 10⁶/L). One concerning adverse effect noted in this study was significant lymphopenia, which developed in 29 of the 237 women receiving ipriflavone. The largest study, the Ipriflavone Multicenter European Fracture Study, revealed a significant incidence of lymphopenia in the group receiving ipriflavone, compared with placebo. In most cases lymphopenia resolved within 12 to 24 months of discontinuation of the supplement.
Of the 29 cases of lymphocytopenia that developed during ipriflavone treatment, 15 (52%) had recovered spontaneously by 1 year and 22 (81%) by 2 years after cessation.
8.2 Gastrointestinal Effects
Excluding the Ipriflavone Multicenter European Fracture Study, pooled results from 2,769 patients participating in small clinical trials revealed adverse effects in 14.5% of patients receiving ipriflavone and in 16.1% of women taking placebo. Complaints in both groups were primarily gastrointestinal: heartburn, vomiting, abdominal pain, constipation, and diarrhea.
8.3 CYP450 Enzyme Inhibition and Drug Interactions
The effects of ipriflavone and its major metabolites 7-hydroxy-isoflavone and 7-(1-carboxy-ethoxy)-isoflavone on cytochrome P450 activities were studied in vitro in human liver microsomes. Ipriflavone and 7-hydroxy-isoflavone competitively inhibited phenacetin O-deethylase and tolbutamide hydroxylase activity. The parent compound and its dealkylated metabolite were strong inhibitors exhibiting Ki values around 10–20 μM, while 7-(1-carboxy-ethoxy)-isoflavone had no effect on the cytochrome P450 activities investigated.
Theophylline interaction: Ipriflavone might decrease how quickly the body eliminates theophylline. Taking ipriflavone along with theophylline might increase the effects and side effects of theophylline. Research suggests that the decrease in theophylline metabolism and increasing levels of serum theophylline observed during ipriflavone administration may be related to CYP3A inhibition by ipriflavone. A clinical case report (Takahashi et al., Eur J Clin Pharmacol 1992) described elevation of serum theophylline levels in a patient with chronic obstructive pulmonary disease who was co-administered ipriflavone.
Warfarin and anticoagulants: Ipriflavone should be used with caution in patients with liver or kidney disease and in those taking theophylline. Increased anticoagulant activity was noted when acenocoumarol was administered with ipriflavone; therefore, caution is also indicated with use of this agent.
8.4 Renal Insufficiency
Dosage reduction is recommended in patients with renal insufficiency.
8.5 Liver Disease
Since ipriflavone is metabolized by the liver, those with liver disease are advised to avoid it unless directed otherwise by a healthcare professional.
8.6 General Safety Profile
The 2020 systematic review and meta-analysis concluded that gastrointestinal symptoms may occur with ipriflavone, but adverse drug withdrawal events were not statistically increased when compared with the placebo group. Ipriflavone is rapidly metabolized and quickly eliminated, therefore it cannot cause any long-acting side effects, and patients' compliance is generally good. However, the lymphocytopenia observed in the IMEFS represents a meaningful and documented safety concern requiring monitoring during prolonged use.
9. Overall Evidence Summary
Ipriflavone has been investigated in well over 150 human and animal studies since the 1980s, primarily in Italy, Japan, and Hungary. The preponderance of smaller randomized controlled trials conducted through the 1990s, which were largely positive for BMD preservation, must be balanced against the IMEFS, the largest and most rigorously conducted multicenter trial, which found no statistically significant effect of ipriflavone on BMD, biochemical markers of bone turnover, or vertebral fracture incidence over 3 years. The 2020 systematic review and meta-analysis concluded that IP significantly increases BMD and has an inhibitory effect on bone resorption markers in postmenopausal women with osteopenia or osteoporosis, yet this conclusion is based on heterogeneous trial populations, and the controversy created by the null IMEFS result has not been fully resolved. For Paget's disease, renal osteodystrophy, and drug-induced bone loss, evidence is preliminary and based on small early-phase studies only. The lymphocytopenia signal identified in the IMEFS is a documented and clinically meaningful concern for long-term use at therapeutic doses.
References
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