Alcaloides oxindólicos

Oxindole Alkaloids

1. Identity: Chemical and Botanical Classification

Oxindole alkaloids are a chemically defined class of naturally occurring nitrogen-containing secondary metabolites characterized by the presence of an oxindole moiety — a bicyclic structure containing a benzene ring fused with a five-membered lactam ring carrying a carbonyl group (N–C=O). They are a monoterpene group of alkaloids that exhibit an oxindole moiety (N–C=O) in ring B of the structure, with either corynantheoid or ajmalicinoid analogues exhibiting tetracyclic or pentacyclic oxindole skeletons. This structural nucleus confers on the alkaloids a wide range of pharmacological properties and makes the oxindole scaffold one of the more intensively studied cores in medicinal chemistry.

Oxindole has been shown to be a pharmacologically advantageous scaffold having many biological properties relevant to medicinal chemistry, with its value further increased by its natural occurrence as alkaloids in a variety of plants. Oxindole, a bicyclic monoterpene alkaloid, forms the core of a class of biologically important compounds; being a tryptophan derivative, it is also produced in the human body in the gut by normal-flora bacteria, earning it the name "human metabolite of indole."

Within the oxindole alkaloid class, two principal skeletal sub-types are recognized based on ring-count:

• Pentacyclic oxindole alkaloids (POA): contain five fused rings and include isopteropodine, pteropodine, mitraphylline, isomitraphylline, speciophylline, and uncarine F. POAs affect the immunocellular system, increasing the rate of phagocytosis by granulocytes and inducing lymphocyte specificity.
• Tetracyclic oxindole alkaloids (TOA): contain four fused rings and include rhynchophylline, isorhynchophylline, corynoxeine, and isocorynoxeine. Tetracyclic oxindole alkaloids act on the central nervous system; for instance, isorhynchophylline can improve memory problems by increasing antioxidant levels, while D-galactose exerts an anti-inflammatory effect on brain tissues in mice.

The occurrence of this group of alkaloids in Uncaria species may be derived from the transition of heteroyohimbines (indole alkaloids) via indolenine intermediates brought about by differences in cell pH values.

Principal Botanical Sources

The primary botanical source of oxindole alkaloids is the genus Uncaria (family Rubiaceae). This group of alkaloids, which is prevalent in Uncaria species but also exists in the related Mitragyna species, has been largely explored in terms of their biological and pharmacological potential, in particular immunomodulatory and CNS-related properties. There are at least 36 species referred to as Uncaria.

The most commercially and medically prominent species include:

• Uncaria tomentosa (Willd. ex Schult.) DC. — the Peruvian or South American cat's claw; the dominant species in North American commerce and the subject of most clinical research. It is a tropical medicinal vine originating at the Amazon rainforest and other areas of South and Central America.
• Uncaria guianensis (Aubl.) Gmel. — the second South American cat's claw species, more common in some European markets. Both species are present in the Brazilian Amazon, are popularly known as "cat's claw," and are prolific producers of bioactive spirocyclic oxindole alkaloids.
• Uncaria rhynchophylla (Miq.) Miq. ex Havil. — the East Asian "Gouteng" or "Uncaria Hook," used in Traditional Chinese Medicine (TCM). Rhynchophylline, a major tetracyclic oxindole alkaloid in U. rhynchophylla, is present in a plant that has been extensively used as a traditional herbal medicine for the prevention of convulsions and hypertension.
• Uncaria sinensis (Oliv.) Havil. — another TCM species. Five new 16-carboxy derivatives of oxindole alkaloids — isopteropodic acid, pteropodic acid, mitraphyllic acid, isorhynchophyllic acid, and rhynchophyllic acid — have been identified from its hooks and stems.

Many reports exist on the chemical composition of the various Uncaria species. More than 150 chemical compounds have been isolated from the genus, with approximately 50 chemical entities attributed to U. tomentosa, some of which are possibly novel to that species.

Common Names and Synonyms

Uncaria tomentosa is known as "uña de gato," "samento," "minsho-mentis," and "garabato," and is revered as "the life-giving vine of Peru." It is also sold commercially under proprietary names including C-Med-100, Reparagen, Saventaro, and Krallendorn. It is widely known as cat's claw or "unha de gato" in Peruvian and Brazilian traditional medicine.

Forms and Preparations

Tinctures, decoctions, capsules, extracts, and teas are prepared from cat's claw. The extract is usually made from the root bark, most commonly from U. tomentosa. Oxindole alkaloids occur throughout the plant, but concentrations vary significantly by organ and season. There is increasing demand for medicinal extracts with biological activity attributed mainly to oxindole alkaloids, where the ratio between tetracyclic (TOA) and pentacyclic (POA) forms determines feasibility for medicinal applications; this ratio is affected by distinct factors including the dynamics of environmental conditions during seasons.

A critical quality consideration is the existence of two distinct chemotypes of U. tomentosa: one dominated by POAs and one dominated by TOAs. The two chemical types contain either tetracyclic oxindole alkaloids (TOA) or pentacyclic oxindole alkaloids (POA). In Western Europe, only the TOA-free form has been licensed for the treatment of rheumatoid arthritis (Krallendorn, Immodal Pharmaka, Austria), because data demonstrated that only POAs stimulate the immune system, whereas TOAs antagonize this effect. No official standardization exists, and commercially available preparations may consist of mixed plant sources despite claiming to be "100% pure."

2. Traditional and Historical Use

Uncaria tomentosa has been used for at least 2,000 years among some Peruvian tribes, especially the Asháninka. The plant has been used medicinally by the Aguaruna, Ashaninka, Cashibo, Conibo, and Shipibo tribes of Peru for at least 2,000 years.

Cat's claw has a long history within the indigenous tribes of the Amazon River basin, where the bark and roots were prepared as a decoction to treat various ailments. Ethnic groups in the Peruvian Amazon, such as the Ashaninka, Aguaruna, Huambisa, Jivaro, Shipibo-Conibo, Bora, Mashiguenga, Campa, and Piro tribes have utilized cat's claw for centuries to treat conditions including arthritis, allergies, asthma, diabetes, cancer, and bacterial and viral infections.

The Ashaninka specifically used cat's claw to treat asthma, inflammations of the urinary tract, arthritis, rheumatism, and bone pain; to recover from childbirth; as a kidney cleanser; to cure deep wounds; to control inflammation and gastric ulcers; and for cancer. Indigenous tribes in Piura used cat's claw to treat tumors, inflammations, rheumatism, and gastric ulcers.

Cat's claw was also used as a medicinal plant by the ancient Incas. It is widely used in Peruvian and Brazilian traditional medicine as an anti-inflammatory, antinociceptive, and antiasthmatic agent, and to prevent diseases. Reflecting this longstanding role, U. tomentosa is included in the National List of Essential Medicines (RENAME) provided by the Brazilian Ministry of Health to all municipalities through the national health system (SUS).

A decoction of U. tomentosa bark, root, and leaves is used traditionally for different health problems, including arthritis, weakness, viral infections, skin disorders, abscesses, allergies, asthma, cancer, fevers, gastric ulcers, haemorrhages, inflammations, menstrual irregularity, rheumatism, urinary tract inflammation, and wounds, among others, which gave rise to scientific and commercial interest.

In the Peruvian–Amazonian tradition, plant use forms part of a broader healing framework. In Peruvian–Amazonian traditional medicine, a medicinal plant is used as part of la dieta — a focused retreat of the person with the ailment that involves adhering to dietary, social, and behavioural limitations along with the application of an appropriate herb such as cat's claw.

In East Asian medicine, species of Uncaria have a separate and parallel history. Uncaria hook (UH) contains several indole and oxindole alkaloids including corynoxeine, isocorynoxeine, rhynchophylline, isorhynchophylline, hirsuteine, hirsutine, and geissoschizine methyl ether, which have similar pharmacological neuroprotective effects against Alzheimer's disease, Parkinson's disease, and depression. UH has therapeutic effects on ailments of the cardiovascular and central nervous systems.

3. Key Constituents and Active Compounds

U. tomentosa is rich in many phytoconstituents such as oxindole and indole alkaloids, glycosides, organic acids, proanthocyanidins, sterols, and triterpenes. However, the oxindole alkaloids are regarded as the primary bioactive fraction for immunological applications.

Pentacyclic Oxindole Alkaloids (POAs)

Constituents of cat's claw extracts include oxindole alkaloids — specifically isopteropodine, pteropodine, rhynchophylline, mitraphylline, speciophylline, uncarine F, and uncarine E — as well as indole alkaloidal glucosides (cadambine, 3-dihydrocadambine, and 3-isodihydrocadambine), quinovic acid glycosides, tannins, polyphenols, catechins, and beta sitosterol.

The six principal pentacyclic oxindole alkaloids are isopteropodine (uncarine E), pteropodine (uncarine C), mitraphylline, isomitraphylline, speciophylline (uncarine D), and uncarine F. Mitraphylline and isopteropodine are considered the chemical markers of the U. tomentosa species according to the USP (2023). The POA mitraphylline and isopteropodine are the chemical markers used in the quality control of cat's claw herbal medicine.

Tetracyclic Oxindole Alkaloids (TOAs)

The principal TOAs are rhynchophylline and isorhynchophylline. These are the dominant alkaloids in Asian Uncaria species such as U. rhynchophylla and U. sinensis, and they occur as minor alkaloids in South American U. tomentosa. The alkaloids studied by solid-state NMR and theoretical GIAO DFT methods in U. tomentosa include mitraphylline, isomitraphylline, pteropodine (uncarine C), isopteropodine (uncarine E), speciophylline (uncarine D), rhynchophylline, and isorhynchophylline, which are 7R/7S and 20R/20S stereoisomeric pairs.

Alkaloid Content Variability

Variations in the total alkaloid content in commercial products range from 0.156 to 0.962%. In an aqueous extract of U. tomentosa bark, oxindole alkaloids were the most abundant compounds at 19.66 ± 0.01 μg/mL, followed by low molecular weight polyphenols at 13.25 ± 0.07 μg/mL and quinovic acid derivatives at 7.46 ± 0.01 μg/mL.

4. Established Mechanisms of Action

4.1 Immunomodulation

Reference to the use of alkaloids isolated from U. tomentosa goes as far back as 1985, when Wagner and colleagues observed that four out of six oxindole alkaloids present in the plant caused a pronounced enhancement of phagocytosis, both in vitro and in vivo. POAs affect the immunocellular system, increasing the rate of phagocytosis by granulocytes and inducing lymphocyte specificity.

The immunological activity is highly dependent on chemotype. It has been recognized that tetracyclic alkaloids can significantly reduce activity of pentacyclic alkaloids, and according to the U.S. Pharmacopeia, only extracts free of tetracyclic oxindole alkaloids may be used in humans for research and/or therapeutic purposes.

4.2 Anti-Inflammatory Activity

These alkaloids have garnered interest due to their reported immunomodulatory and anti-inflammatory effects. Several in vitro and animal studies have demonstrated that tetracyclic oxindole alkaloids can inhibit the production of pro-inflammatory cytokines (such as TNF-α and IL-1β) and modulate immune cell signaling pathways.

Researchers have documented a wide array of pharmacological properties including limitation of epithelial cell death in response to oxidant stress, amelioration of oedema via inhibition of cyclooxygenase-1 and cyclooxygenase-2, cytoprotection by means of free radical scavenging, reduction of oxidative stress, and direct inhibition of TNF-α production.

4.3 Anticancer / Pro-Apoptotic Activity

Oxindole alkaloids are particularly potent in their anticancer activities. They can induce apoptosis, inhibit cell proliferation, and exhibit cytotoxic effects against various cancer cell lines, making them valuable in the search for effective cancer therapies.

In cell research on human lymphoblastic leukaemia T cells (CCRF-CEM-C7H2), the antiproliferative and apoptotic effects of highly purified oxindole alkaloids — isopteropodine, pteropodine, isomitraphylline, uncarine F, and mitraphylline — obtained from Uncaria tomentosa were investigated. Four of the five tested alkaloids inhibited proliferation of acute lymphoblastic leukaemia cells, and the antiproliferative effect of the most potent alkaloids pteropodine and uncarine F correlated with induction of apoptosis. Neither overexpression of bcl-2 or crm-A nor cell-cycle arrest in G0/G1 phase could prevent alkaloid-induced apoptosis.

4.4 Neuroprotective Mechanisms

Isorhynchophylline has been reported to exert anti-oxidative effects, stabilize mitochondrial membrane potential, and inhibit tau protein hyperphosphorylation, all contributing to neuroprotective effects.

Rhynchophylline (10–50 μM) greatly prevented neurotoxicity caused by 1-methyl-4-phenylpyridinium ion (MPP+) in primary cerebellar granule neurons, as evidenced by the promotion of cell viability as well as the reversal of dysregulated protein expression of Bax/Bcl-2 ratio.

Isorhynchophylline promoted clearance of wild-type, A53T and A30P α-synuclein monomers, α-synuclein oligomers, and α-synuclein/synphilin-1 aggresomes in neuronal cells via the autophagy-lysosome pathway. More importantly, it was able to decrease the expression levels of wild-type and A53T α-synuclein protein in differentiated human dopaminergic neurons.

4.5 DNA Repair Enhancement

Repair of DNA single strand breaks (SSB) and double strand breaks (DSB) three hours after 12 Gy whole-body irradiation of rats was significantly improved in C-MED-100 (a water-soluble extract of U. tomentosa) treated animals (p < 0.05). Phytohemagglutinin-stimulated lymphocyte proliferation was significantly increased in splenocytes of rats treated at the doses of 40 and 80 mg/kg; white blood cells from the C-MED-100 treatment groups of 40 and 80 mg/kg for 8 weeks or 160 mg/kg for 4 weeks were significantly elevated compared with controls (p < 0.05).

4.6 Drug Transporter Interactions

A 2024 study found that U. tomentosa extract significantly inhibited all ABC transporters and the majority of the SLC transporters tested. Of the investigated oxindole alkaloids, isopteropodine significantly inhibited OATP, OCT1 and OCT2, OAT3, ENT4, MDR1, and BCRP transporters. This has potential implications for drug–herb interactions (discussed further in the safety section).

5. Scientific Evidence by Area of Use

5.1 Rheumatoid Arthritis (RA)

Evidence strength: Preliminary; limited human data.

In a double-blind, placebo-controlled trial of forty individuals undergoing conventional treatment for rheumatoid arthritis, use of an extract made from U. tomentosa modestly improved symptoms compared with placebo. The researchers used the form of the plant containing pentacyclic oxindole alkaloids, as opposed to tetracyclic oxindole alkaloids.

In that randomized trial of cat's claw (30 mg daily) versus placebo in 40 patients with rheumatoid arthritis, adverse events were uncommon and minor, the most common being dyspepsia and pruritus; laboratory results did not change. The trial was published in the Journal of Rheumatology (Mur et al., 2002). Despite positive findings, the small sample size and single-study status limit conclusions.

5.2 Osteoarthritis

Evidence strength: Preliminary; small clinical trials with mixed objective outcomes.

Among 45 patients with painful osteoarthritis of the knee treated with cat's claw (100 mg once daily) or placebo for 4 weeks, pain scores decreased more with cat's claw, but night-time pain and swelling did not change; cat's claw was well tolerated and there were no serious adverse events or changes in serum ALT and AST values during treatment.

Another study compared the effectiveness of a proprietary combination of cat's claw with glucosamine sulfate. Researchers reported positive results, but because there was no placebo group, the overall effectiveness of this cat's claw combination product cannot be determined. More research will be necessary to verify this potential use.

5.3 Immune Function and DNA Repair

Evidence strength: Preliminary; small human volunteer studies.

In a human volunteer study, C-MED-100 was given daily at 5 mg/kg for 6 consecutive weeks to four healthy adult males. No toxicity was observed and white blood cells were significantly elevated (p < 0.05) after supplementation. This is a very small study (n = 4) and its results require replication.

A water-soluble extract from Uncaria tomentosa (cat's claw) was identified as a potent enhancer of DNA repair in primary organ cultures of human skin. (Mammone et al., 2006, published in Phytotherapy Research.) This is an organ-culture study and not a clinical trial.

5.4 Cancer (Adjuvant or Supportive Context)

Evidence strength: Preclinical only; very limited human data.

The most promising anticancer findings were observed for crude aqueous bark extracts (72 h incubation) against squamous cell carcinoma and pentacyclic oxindole alkaloid (POA)-rich extracts against prostate cancer and leukemia. In contrast, tetracyclic oxindole alkaloid (TOA)- and proanthocyanidin (PAC)-rich fractions showed limited cytotoxicity. Most extracts were non-toxic to normal cells, except for the crude aqueous bark extract, which exhibited cytotoxicity in keratinocytes.

U. tomentosa has potential as a source of selective anticancer agents, particularly through crude aqueous bark and POA-rich extracts. The observed cytotoxic effects vary considerably depending on the extraction method and chemical composition, underscoring the need for standardization in future studies. Further standardized studies and mechanistic investigations are required to validate its therapeutic potential for cancer treatment.

One phase II clinical trial (NCT02045719) assessed the effects of a 100-mg dose of a dry extract of U. tomentosa three times per day on individuals with advanced solid tumors with no further therapeutic options. This prospective phase II study assessed the effects of a 100-mg dose of a dry extract of U. tomentosa three times per day on individuals with advanced solid tumors with no further therapeutic options and with at least 2 months life expectancy. The publication record for this trial is limited, and no strong human clinical evidence currently supports the use of oxindole alkaloids as anti-cancer agents.

5.5 Neurological Conditions

Evidence strength: Preclinical only; no published randomized human trials specific to oxindole alkaloids.

The purified oxindole alkaloids isomitraphylline and mitraphylline from Uncaria perrottetii revealed their ability to break amyloid aggregates in vitro, suggesting therapeutic potential in Alzheimer's disease (AD). Thioflavin-T assay results showed Aβ aggregation inhibitions at 60.321% ± 2.61 (50 μM) for isomitraphylline and 43.17% ± 3.48 (50 μM) for mitraphylline. Neuroprotective effects were elaborated against Aβ-induced SH-SY5Y cells at 20 μM and 10 μM for isomitraphylline, and 20 μM for mitraphylline. Both alkaloids also attenuated and protected against H2O2-induced SH-SY5Y cell cytotoxicity at 20 μM.

Uncaria hook contains several indole and oxindole alkaloids — corynoxeine, isocorynoxeine, rhynchophylline, isorhynchophylline, hirsuteine, hirsutine, and geissoschizine methyl ether — which have similar pharmacological neuroprotective effects against Alzheimer's disease, Parkinson's disease, and depression; the mechanisms of these effects include anti-oxidant, anti-inflammatory, and neuromodulatory activities. All of this evidence is from animal models and cell studies; no human clinical trials on oxindole alkaloids for neurodegeneration have been published to date.

5.6 Anti-Inflammatory Applications (General)

Evidence strength: Moderate in preclinical models; limited and indirect in humans.

The quality and quantity of clinical evidence in humans is limited. Most human studies use extracts of cat's claw, which contain a mixture of alkaloids and other compounds, making it difficult to attribute effects solely to tetracyclic oxindole alkaloids. Some small clinical trials and case reports suggest cat's claw may help with symptoms of inflammatory conditions like rheumatoid arthritis, but these studies are generally low-powered and do not isolate the relevant compounds.

A 2024 systematic review and meta-analysis of preclinical studies examined the anti-inflammatory and immunomodulatory activities of U. tomentosa extracts in animal models. The study was conducted to evaluate the effect of U. tomentosa extracts in modulating inflammatory mediators and to determine which types of inflammatory diseases can be treated by this species; researchers conducted a systematic review and meta-analysis of preclinical studies published before 26 July 2023, identified in PubMed, Embase, and Scopus. This review supported preclinical anti-inflammatory activity but underscored the shortage of human trial data.

6. Body Systems and Health Areas

Based on the available published evidence, oxindole alkaloids from Uncaria species have been associated with the following body systems and health areas:

• Immune system: POAs affect the immunocellular system, increasing the rate of phagocytosis by granulocytes and inducing lymphocyte specificity.
• Musculoskeletal system: Cat's claw is used as an analgesic and anti-inflammatory agent to treat gastrointestinal, rheumatologic, and other chronic inflammatory conditions.
• Central nervous system: Tetracyclic oxindole alkaloids act mostly on the central nervous system.
• Cardiovascular system: Uncaria hook contains active indole and oxindole alkaloids and has therapeutic effects on ailments of the cardiovascular and central nervous systems.
• Gastrointestinal system: Traditional use documented for gastric ulcers, gastritis, and digestive complaints. Cat's claw is a Peruvian herbal preparation with several traditional uses including gastritis, rheumatism, cirrhosis, gonorrhea, and cancers of the female genital tract.
• Oncology (research context): The antitumor activity of Uncaria tomentosa, a native vine from the Amazonian rainforest, has been ascribed to pentacyclic oxindole alkaloids occurring in its bark.
• Genomic integrity / DNA repair: Associated with enhancement of DNA repair mechanisms in preclinical and limited human volunteer data (discussed above).

7. Dosage Forms and Reported Dosages

The typical and recommended dose of U. tomentosa is one gram given two to three times daily. A standardized extract consisting of less than 0.5% oxindole alkaloids and 8% to 10% carboxy alkyl esters has been used in doses of 250 to 300 mg in several clinical studies.

One gram of root bark given 2 to 3 times daily is a typical dose, while 20 to 30 mg of a root bark extract has been recommended. Clinical trials are generally lacking to support appropriate dosages. A standardized extract containing 8% to 10% carboxy alkyl esters and less than 0.5% oxindole alkaloids has been used in clinical studies in doses of 250 to 300 mg.

Specific dosages reported in published studies include:

• 30 mg daily (pentacyclic alkaloid-chemotype extract) in a 40-patient rheumatoid arthritis randomized trial (Mur et al., 2002).
• 100 mg once daily in a 45-patient osteoarthritis of the knee trial for 4 weeks (Piscoya et al., 2001).
• 5 mg/kg daily of C-MED-100 for 6 consecutive weeks in a human volunteer study involving four healthy adult males (Sheng et al., 2001).
• 100 mg three times per day of a dry extract in a phase II study of advanced solid tumors (Cruz et al., ClinicalTrials.gov NCT02045719).
• In humans, no toxic symptoms were noticed with frequent administration of 350 mg/day for 6 successive weeks.

In rats, the average lethal dose for a single dose of water extract from U. tomentosa is higher than 8 g/kg.

8. Safety Considerations and Drug Interactions

8.1 General Tolerability

Cat's claw is an herbal medicine used for its anti-inflammatory and immune-modulating effects; it has not been implicated in causing liver injury. Short-term clinical trials report a generally favorable safety profile. In a randomized trial of cat's claw (30 mg daily) versus placebo in 40 patients with rheumatoid arthritis, adverse events were uncommon and minor, the most common being dyspepsia and pruritus; laboratory results did not change.

Although rats were treated daily with U. tomentosa extracts at doses of 10–80 mg/kg for 8 weeks or 160 mg/kg for 4 weeks, no acute or chronic toxicity signs were observed symptomatically; no body weight, food consumption, organ weight, or kidney, liver, spleen, and heart pathological changes were found to be associated with C-MED-100 treatment.

8.2 Chemotype-Related Safety Concerns

Tetracyclic alkaloids can significantly reduce activity of pentacyclic alkaloids, and according to the U.S. Pharmacopeia, only extracts free of tetracyclic oxindole alkaloids may be used in humans for research and/or therapeutic purposes. Because the two chemotypes of U. tomentosa are visually indistinguishable, standardization and laboratory verification of alkaloid profile is a genuine quality and safety concern for commercial preparations.

8.3 Drug Interactions: CYP3A4 and Drug Transporters

Data indicate that extracts of Uncaria tomentosa inhibit the most important drug-metabolizing enzyme cytochrome P450 3A4 (CYP3A4), possibly explaining a published case report describing an interaction between this compound and the HIV protease inhibitors atazanavir, ritonavir, and saquinavir.

The effect of U. tomentosa extract and its major oxindole alkaloids was investigated on multispecific solute carrier (SLC) and ATP-binding cassette (ABC) drug transporters. The extract significantly inhibited all ABC transporters and the majority of the SLC transporters tested. Of the investigated oxindole alkaloids, isopteropodine significantly inhibited OATP, OCT1 and OCT2, OAT3, ENT4, MDR1, and BCRP transporters. The clinical relevance of these transporter inhibition findings in humans requires further investigation.

8.4 Specific Contraindications and Populations

Cat's claw products should be avoided before and after surgery, as well as by those using immunosuppressant therapy and in children due to lack of safety data. Information regarding safety and efficacy during pregnancy and lactation is lacking.

A neurological interaction has been reported in the literature: a case report described reversible worsening of Parkinson's disease motor symptoms after oral intake of Uncaria tomentosa (cat's claw) (Cosentino and Torres, Clinical Neuropharmacology, 2008). This is a single case report and its mechanism remains to be clarified.

8.5 Renal Concerns

Cat's claw is a Peruvian herbal preparation with several traditional uses; however, its use has been associated with the development of acute interstitial nephritis leading to acute renal failure. This finding is based on case reports and has not been systematically quantified.

8.6 Product Quality and Standardization Gaps

Two different chemotypes of U. tomentosa have been described, while U. guianensis and U. rhynchophylla have also been extensively studied. No official standardization exists, and commercially available preparations may consist of mixed plant sources despite claiming to be "100% pure." Inter-batch variations have been reported.

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