Caffeic acid
Caffeic acid, C9H8O4 is a naturally occurring phenolic compound, (formerly called a carbolic acid), which is found in many fruits, vegetables, and herbs, including coffee, although varying in amounts depending on the plant.
Caffeic acid has been shown to act as a carcinogenic inhibitor. It is also known as an antioxidant in vitro and also in vivo.
Caffeic acid is a yellow crystalline acid that is soluble in hot water and alcohol. It is related to cinnamic acid, but it has two hydroxyl groups not found in cinnamic acid. Both are part of the carbocyclic carboxylic acid group.
Caffeic acid can be esterified with quinic acid to form chlorogenic acid. Both caffeic acid and chlorogenic acid are found in coffee beans. Caffeine is a different compound than caffeic acid. A derivative of caffeic acid is caffeic acid phenethyl ester (CAPE).
It is used as a matrix in MALDI mass spectroscopy analyses.

# Biological importance
Caffeic acid and its derivative caffeic acid phenethyl ester (CAPE) are produced in many plants including: pears, basil, thyme, verbena, tarragon, oregano, wood betony, burning bush, turmeric, dandelion, yarrow, horsetail, rosemary, hawthorn and coffee. The amount of caffeic acid is strongly dependent on the plant species.
Both caffeic acid and chlorogenic acid have been shown to be absorbed in humans. Caffeic acid absorption is hampered when it is esterified with quinic acid to form chlorogenic acid.
In laboratory experiments, colonies of a nut tree mould were grown on extracts of walnut and pistachio. Next, fungal colonies were exposed to three compounds thought to be antioxidants: gallic acid, which has aflatoxin-combating impacts in walnuts, and chlorogenic acid and caffeic acid.
Caffeic acid outperformed the other antioxidants, reducing aflatoxin production by more than 95 percent. The studies are the first to show that oxidative stress that would otherwise trigger or enhance Aspergillus flavus aflatoxin production can be stymied by caffeic acid. This opens the door to using natural anti-fungicide methods by supplementing trees with antioxidants. 

# Pharmaceutical Uses
Oral administration of high doses of caffeic acid in rats has caused stomach papillomas, leading to the perception of caffeic acid as carcinogenic. In the same study, only high doses of combined antioxidants, including caffeic acid, showed a significant decrease in growth of colon tumors in those same rats. No significant effect was noted otherwise. .
Caffeic Acid is still listed under older Hazard Data sheets  as a potential carcinogen because of two early experiments on rats and mice. More recent data show that bacteria in the rats' guts may alter the formation of metabolites of Caffeic Acid.  and  There have been no known ill-effects of Caffeic Acid in humans.
Caffeic Acid and its derivative, Caffeic Acid Phenethyl Ester (CAPE) have shown tumor-shrinking properties. When an anti-cancer drug was being sought, Caffeic Acid and CAPE were derived from Burning Bush (Euonymus alatus). "The subcutaneous and oral administrations of CA and CAPE significantly reduced liver metastasis. These results confirm the therapeutic potential of the compounds and suggest that the anti-metastatic and anti-tumor effects of CA and CAPE are mediated through the selective suppression of MMP-9 enzyme activity and transcriptional down-regulation by the dual inhibition of NF-B as well as MMP-9 catalytic activity." 
A study using the caffeic acid phenethyl ester (CAPE) showed a positive effect on reducing carcinogenic incidence. Caffeic acid phenethyl ester (CAPE) is an active component of propolis from honeybee hives. It is known to have antimitogenic, anticarcinogenic, anti-inflammatory, and immunomodulatory properties.
Another study also showed that CAPE suppresses acute immune and inflammatory responses and holds promise for therapeutic uses to reduce inflammation.
This anti-inflammatory and anti-cancer property has also been shown to protect skin cells when exposed to ultraviolet (UV) radiation, in particular UVC radiation  and UVB radiation. . This anti-cancer effect was also seen when mice skin was treated with bee propolis and  exposed to TPA (a chemical) that induced skin papillomas. CAPE significantly reduced the number of papillomas.  
Caffeic Acid and chlorogenic acid from coffee beans both reduced DNA methylation in vitro in two lines of human cancer cells. DNA methylation contributes to the growth of tumors and regulates the epigenetics of cells that are passed along with DNA to future generations .