Irinotecan
Irinotecan, sold under the brand name Camptosar among others, is a medication used to treat colon cancer, and small cell lung cancer. For colon cancer it is used either alone or with fluorouracil. For small cell lung cancer it is used with cisplatin. It is given intravenously. Common side effects include diarrhea, vomiting, bone marrow suppression, hair loss, shortness of breath, and fever. Other severe side effects include blood clots, colon inflammation, and allergic reactions. Those with two copies of the UGT1A1*28 gene variant are at higher risk for side effects. Use during pregnancy can result in harm  to the baby. Irinotecan is a topoisomerase inhibitor -- it blocks the topoisomerase 1 enzyme, resulting in DNA damage and cell death. Irinotecan was approved for medical use in the United States in 1996. It is on the World Health Organization's List of Essential Medicines. It is made from the natural compound camptothecin which is found in the Chinese ornamental tree "Camptotheca acuminata". Medical uses. Its main use is in colon cancer, in particular, in combination with other chemotherapy agents. This includes the regimen FOLFIRI, which consists of infusional 5-fluorouracil, leucovorin, and irinotecan. The regimen XELIRI consists of capecitabine  and irinotecan. It may also be used together with fluorouracil and folinic acid for pancreatic cancer following failure of initial treatment. Side effects. The most significant adverse effects of irinotecan include diarrhea, nausea and vomiting, neutropenia and fever, infections of blood or lungs (sepsis, pneumonia), shock, dehydration, kidney failure and thrombocytopenia (low levels of blood platelets). Diarrhea. Irinotecan-associated diarrhea is severe and clinically significant, sometimes leading to severe dehydration requiring hospitalization or intensive care unit admission. This side-effect is managed with the aggressive use of antidiarrheals such as loperamide or atropine with the first loose bowel movement. Immunosuppression . The immune system is adversely impacted by irinotecan. This is reflected in – sometimes dramatically – lowered white blood cell counts in the blood, in particular the neutrophils. The patient may experience neutropenia (a clinically significant decrease of neutrophils in the blood). Mechanism of action. Camptothecin, one of the four major structural classifications of plant-derived anti-cancerous compounds, is a cytotoxic alkaloid which consists of a pentacyclic ring structure containing a pyrrole (3, 4 β) quinoline moiety, an S-configured lactone form, and a carboxylate form. Irinotecan is activated by hydrolysis to SN-38, an inhibitor of topoisomerase I. This is then  inactivated by glucuronidation by uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1). The inhibition of topoisomerase I by the active metabolite SN-38 eventually leads to inhibition of both DNA replication and transcription. The molecular action of irinotecan occurs by trapping a subset of topoisomerase-1-DNA cleavage complexes, those with a guanine +1 in the DNA sequence. One irinotecan molecule stacks against the base pairs flanking the topoisomerase-induced cleavage site and poisons (inactivates) the topoisomerase 1 enzyme. Pharmacokinetics. Administration. Irinotecan can be administrated by 30- or 90-minute intravenous infusions of either 125 mg/m weekly for four of every six weeks or  350 mg/m every three weeks. Distribution. Irinotecan is a hydrophilic compound with a large volume of distribution (400 L/m). At physiological pH, irinotecan and its active metabolite ethyl-10-hydroxy-camptothecin (SN-38) are present in two pH-dependent equilibrium isoforms; the anti tumor active lactone ring which hydrolyzed to the carboxylate isoform. In plasma, the majority of irinotecan and SN-38 are bound to albumin, which stabilizes their lactone forms. In blood, irinotecan and SN-38 are bound to platelets and red blood cells. Irinotecan has a linear pharmacokinetic. Population pharmacokinetic models assumed a three-compartmental model for irinotecan and a  two-compartmental model for SN-38. SN-38 has a short distribution half-life (approximately 8 min). It reached its peak plasma concentration within 2 h after infusion. Also SN-38 exhibit a second peak in the plasma concentration because of its enterohepatic re-circulation and its release from erythrocytes. Metabolism. Activation by carboxylesterases and butyrylcholinesteras. About 2–5% of the pro-drug irinotecan is hydrolyzed into its active metabolite SN-38 in the liver by two carboxylesterase converting enzymes (CES1 and CES2) and in plasma by butyrylcholinesterase (hBChE). CES2 has a 12.5-fold higher affinity for irinotecan than CES1. While, butyrylcholinesterase has a 6 -fold higher activity for irinotecan than CES. After conversion, SN-38 is actively transported to the liver by the organic anion transporting polypeptide (OATP) 1B1 transporter. Inactivation by uridine diphosphate glucuronosyltransferases. SN-38 is inactivated by glucuronidation to SN-38G (β-glucuronide conjugate) by several uridine diphosphate glucuronosyltransferase enzymes (UGTs) in the liver (UGT1A1, UGT1A9) and extra-hepatic (UGT1A1, UGT1A7, UGT1A10) and excreted into the bile. Several UGT polymorphisms affects irinotecan pharmacokinetics, for example, the decreased UGT1 activity, may lead to severe toxicity. Also, UGT1A1 conjugates bilirubin and bilirubin glucuronidation is another risk factor for increased toxicity De-conjugation by β-glucuronidases . The intestinal bacteria produced β-glucuronidases that de-conjugate SN-38G to SN-38 resulting in entero-hepatic re-circulation of SN-38. Metabolism by cytochrome P450 enzymes. Irinotecan is metabolized by intrahepatic cytochrome P450 enzymes, CYP3A4 and CYP3A5 into inactive metabolites APC (7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino] carbonyloxycamptothecin) and NPC (7-ethyl-10-[4-amino-1-piperidino] carbonyloxycamptothecin). NPC can be further converted by CES1 and CES2 in the liver to SN-38. Induction or inhibition of CYP3A enzymes by smoking, some herbs and medications may result in interactions with irinotecan. Transport to bile. Irinotecan is transported to bile by the ATP-binding cassette  (ABC) transporter proteins: ABCB1, ABCC1, ABCC2, and ABCG2. Elimination. Irinotecan clearance is mainly biliary (66%) and estimated 12–21 L/h/m. All metabolites, except SN-38G, are mainly excreted in feces. Irinotecan  elimination half-lives were reported between 5 to18 h. SN-38 half-lives were reported between  6 to 32 h. There is high (30%) interindividual variability in irinotecan pharmacokinetic parameters which can be altered by several factors including age, sex, dose, administration timing, hepatic function, enzyme activity or hematocrit levels. Pharmacogenomics. Irinotecan is converted by an enzyme into its active metabolite SN-38, which is in turn inactivated by the  enzyme UGT1A1 by glucuronidation. Section::::*28 variant patients. People with variants of the UGT1A1 called TA, also known as the "*28 variant", express fewer UGT1A1 enzymes in their liver and often have Gilbert's syndrome. During chemotherapy, they effectively receive a larger than expected dose because their bodies are not able to clear irinotecan as fast as others. In studies this corresponds to higher incidences of severe neutropenia and diarrhea. In 2004, a clinical study was performed that both validated prospectively the association of the *28 variant with greater toxicity and the ability of genetic testing in predicting that toxicity before  chemotherapy administration. In 2005, the FDA made changes to the labeling of irinotecan to add pharmacogenomics recommendations, such that irinotecan recipients with a homozygous (both of the two gene copies) polymorphism in UGT1A1 gene, to be specific, the *28 variant, should be considered for reduced drug doses. Irinotecan is one of the first widely used chemotherapy agents that is dosed according to the recipient's genotype. Society and culture. Approval. Irinotecan received accelerated approval from the U.S. Food and Drug Administration (FDA) in 1996, and full approval in 1998. Names. During development, it was known as CPT-11. Formulations. A liposome  encapsulated version of irinotecan sold as Onivyde by Merrimack Pharmaceuticals, was approved by FDA in October 2015, to treat metastatic pancreatic cancer. It was approved for medical use in the European Union in October 2016.