A CDK inhibitor is a chemical that inhibits the function of CDK

Cyclin-dependent kinase

thumb|350px|Schematic of the cell cycle. outer ring: I=[[Interphase]], M=[[Mitosis]]; inner ring: M=Mitosis; G1=[[G1 phase|Gap phase 1]]; S=[[S phase|Synthesis]]; G2=[[G2 phase|Gap phase 2]]...

s. It is used to treat cancer

Cancer

Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

s by preventing overproliferation of cancer cell

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

s. Although there is no approved anti-cancer drugs that target CDKs yet, several compounds are on clinical trials now (2009).

CDKs as cancer target

In many human cancers, CDKs are overactive or CDK-inhibiting proteins are not functional. Therefore, it is rational to target CDK function to prevent unregulated proliferation of cancer cells.

However, the validity of CDK as a cancer target should be carefully assessed because genetic studies have revealed that knockout of one specific type of CDK often does not affect proliferation of cells or has an effect only in specific tissue

Tissue (biology)

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

 types. For example, most adult cells in mice proliferate normally even without both CDK4 and CDK2.

Furthermore, specific CDKs are only active in certain periods of the cell cycle. Therefore, the pharmacokinetics and dosing schedule of the candidate compound must be carefully evaulated to maintain active concentration of the drug throughout the entire cell cycle.

Types of CDK inhibitors

Malumbres et al., categorized CDK inhibitors based on their target specificity;

• Broad CDK inhibitors ; compounds targeting a broad spectrum of CDKs
• Specific CDK inhibitors ; compounds targeting a specific type of CDK
• Multiple target inhibitors ; compounds targeting CDKs as well as additional kinases such as VEGFR or PDGFR

CDK inhibitors on clinical trials

There are more than 10 CDK inhibitor compounds that have gone through or currently ongoing clinical trials, as of 2009. Most of them are targeting multiple CDKs, but some are targeting specific CDKs. For example, P1446A-05 targets CDK4 and PD-0332991 targets CDK4 and CDK6. All compounds are (as of 2009) either in phase I or phase II trials. Various types of cancers including leukemia

Leukemia

Leukemia or leukaemia is a type of cancer of the blood or bone marrow characterized by an abnormal increase of immature white blood cells called "blasts". Leukemia is a broad term covering a spectrum of diseases...

, melanoma

Melanoma

Melanoma is a malignant tumor of melanocytes. Melanocytes are cells that produce the dark pigment, melanin, which is responsible for the color of skin. They predominantly occur in skin, but are also found in other parts of the body, including the bowel and the eye...

, solid tumors, and other types are being targeted. In some cases, very specific cancer types, such as 'melanoma positive for cyclin D1 expression' are targeted to maximize the efficacy. Further details can be found in Lapenna and Giordano or can be searched from http://www.clinicaltrials.gov