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Gemcitabine


ACCESSION NB: DB00441 (APRD00201)


TYPE: small molecule


GROUP: approved


DESCRIPTION:
Gemcitabine is a nucleoside analog used as chemotherapy. It is marketed as Gemzar® by Eli Lilly and Company. As with fluorouracil and other analogues of pyrimidines, the drug replaces one of the building blocks of nucleic acids, in this case cytidine, during DNA replication. The process arrests tumor growth, as new nucleosides cannot be attached to the “faulty” nucleoside, resulting in apoptosis (cellular “suicide”). Gemcitabine is used in various carcinomas: non-small cell lung cancer, pancreatic cancer, bladder cancer and breast cancer. It is being investigated for use in oesophageal cancer, and is used experimentally in lymphomas and various other tumor types.

VOLUME OF DISTRIBUTION: 50 L/m^2 [infusions lasting <70 minutes] 370 L/m^2 [long infusions]

CATEGORIES:
Antineoplastic Agents Antiviral Agents Radiation-Sensitizing Agents Antimetabolites Enzyme Inhibitors Immunosuppressive Agents Antimetabolites, Antineoplastic

ABSORPTION: 100%

INDICATION:
For the first-line treatment of patients with metastatic breast cancer, locally advanced (Stage IIIA or IIIB), or metastatic (Stage IV) non-small cell lung cancer and as first-line treatment for patients with adenocarcinoma of the pancreas.

PHARMACODYNAMICS:
Gemcitabine is an antineoplastic anti-metabolite. Anti-metabolites masquerade as purine or pyrimidine - which become the building blocks of DNA. They prevent these substances becoming incorporated in to DNA during the "S" phase (or DNA synthesis phase of the cell cycle), stopping normal development and division. Gemcitabine blocks an enzyme which converts the cytosine nucleotide into the deoxy derivative. In addition, DNA synthesis is further inhibited because Gemcitabine blocks the incorporation of the thymidine nucleotide into the DNA strand.

MECHANISM OF ACTION:
Gemcitabine inhibits thymidylate synthetase, leading to inhibition of DNA synthesis and cell death. Gemcitabine is a prodrug so activity occurs as a result of intracellular conversion to two active metabolites, gemcitabine diphosphate and gemcitabine triphosphate by deoxycitidine kinase. Gemcitabine diphosphate also inhibits ribonucleotide reductase, the enzyme responsible for catalyzing synthesis of deoxynucleoside triphosphates required for DNA synthesis. Finally, Gemcitabine triphosphate (diflurorodeoxycytidine triphosphate) competes with endogenous deoxynucleoside triphosphates for incorporation into DNA.

PROTEIN BINDING:
Plasma protein binding is negligible (<10%)

METABOLISM:
Transformed via nucleoside kinases to two active metabolites, gemcitabine diphosphate and gemcitabine triphosphate. Can also undergo deamination via cytidine deaminase to an inactive uracil metabolite (dFdU).

TOXICITY:
Myelosuppression, paresthesias, and severe rash were the principal toxicities, LD50=500 mg/kg (orally in mice and rats)

AFECTED ORGANISMS:
Humans and other mammals