Cat's claw (Uncaria tomentosa) is a plant found in the Amazon River basin.1  It gets its name from the curved, hooked thorns present on the vine which resemble a cat's claw.1  It has been used for centuries in South America for conditions such as chronic inflammatory disorders, viral infections, arthritis, gastrointestinal illnesses, cancer and even as birth control.1-4   As it relates to contraception, the safety and efficacy of Cat's claw is limited, but there are reports that if consumed in large amounts (dose not known) at the time of menstruation it can potentially cause sterility for 3 to 4 years with one dose.5  Despite this claim about its use, it is not recommended for use during pregnancy. 

What is the mechanism of action of Cat's claw in relation to its use as a contraceptive and reason to avoid during pregnancy?
There are several proposed mechanisms that are linked to its use for the above indications, but as it relates to contraception and use in pregnancy, it is likely the alkaloids present in Cat's claw.   The number of alkaloids are extensive, but some have been identified as being isopteropodine, isorhynchophylline, isomitraphylline, isocorynoxeine, mitaphylline, pteropodine, rhynchophyline, and speciophylline.1,5  It is alkaloids such as these in Cat's claw that have been shown to have antiproliferative effects in breast cancer cells, leukemic and lymphoma cell lines.6,7  It appears that the antiproliferative effects are not cytogenic or mutagenic, but rather are through an induction of apoptosis (programmed cell death).7-9  It is likely for this reason that Cat's claw has been used as a treatment for cancer - where apoptosis is largely inhibited.1,5 

Why could Cat's claw be used as a contraceptive, but not used during pregnancy?
While we are not aware of any definitive data, it is plausible that the contraceptive effects of Cat's claw are related to its antiproliferative effects and induction of apoptosis.  It is important to note that this effect has been studied in the context of cancer, not contraception.  Nevertheless, given this known mechanism, it is conceivable that while conception may still be taking place, the fetus may be aborted due to the failure of cellular replication and/or differentiation.  This would be especially true if exposure to Cat's claw were to occur during the first 8 to 9 weeks of gestation.  In addition, it is possible that Cat's claw could affect follicle development to the point that ovulation does not result in the release of a viable oocyte.  A review of the literature did not provide any documented mechanisms for either of the above.  However, other plant alkaloid based chemotherapeutic agents (such as paclitaxel and vincristine) are known to be pregnancy category D due to fetal malformations noted in animal studies. 

Therefore, due to the known constituents present in Cat's claw, their known antiproliferative effects on various cancer cell lines, other plant alkaloid based chemotherapeutic agents known to cause fetal malformations in animal studies and the lack of safety or efficacy studies for these situations, it is advisable that females of child-bearing age who are sexually active or pregnant women should avoid the use of Cat's claw regardless of its desired use.

References:

1. Valerio LG Jr, Gonzales GF.  Toxicological aspects of the South American herb's cat's claw (Uncaria tomentosa) and Maca (Lepidium meyenil): a critical synopsis.  Toxicol Rev  2005;24:11-35.
2. Williams JE.  Review of antiviral and immunomodulating properties of plants of the Peruvian rainforest with a particular emphasis on Una de Gato and Sangre de Grado.  Altern Med Rev  2001;6:567-79.
3. Piscoya J, Rodriguez Z, Bustamante SA et al.  Efficacy and safety of freeze-dried cat's claw in osteoarthritis of the knee: mechanism of action of the species Uncaria guyanensis.  Inflamm Res  2001;50:442-8.
4. Mur E, Hartig F, Eibl G et al.  Randomized double blind trial of an extract from the pentacyclic alkaloid-chemotype of Uncaria tomentosa for the treatment of rheumatoid arthritis.  J Rheumatol  2002;29:678-81.
5. Physicians' Desk Reference for Herbal Medicines. 4^th edition.  Cat's Claw.  Thompson Healthcare Inc. 2007:168-70.
6. Riva L, Coradini D, Di Fronzo G et al.  The antiproliferative effects of Uncaria tomentosa extracts and fractions on the growth of breast cancer cell line.  Anticancer Res  2001;21:2457-61. 
   
7. Sheng Y, Pero RW, Amiri A et al.  Induction of apoptosis and inhibition of proliferation in human tumor cells treated with extracts of Uncaria tomentosa.  Anticancer Res  1998;18:3363-8.
   
8. Santa Maria A, Lopez A, Diaz MM et al.  Evaluation of the toxicity of Uncaria tomentosa by bioassays in vitro.  J Ethnopharamacol  1997;57:183-7.
   
9. Rizzi R, Re F, Bianchi A et al.  Mutagenic and antimutagenic activities of Uncaria tomentosa and its extracts.  J Ethnopharmacol  1993;38:63-77.

Cats claw, Uncaria tomentosa, Pregnancy, Cats Claw Use in Pregnancy