Apixaban Pharmacology, Medicinal Chemistry and Drug Design in Relation to a Patient Case"
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Apixaban (Eliquis) is a novel anticoagulant with indications for deep vein thrombosis (DVT), pulmonary embolism (PE), atrial fibrillation, and prophylactic risk reduction of recurrence. While on a retail rotation at Walgreens, a patient was found with two drug interactions involving apixaban, which were ibuprofen and fluvoxamine. The patient was taking the ibuprofen as needed and the fluvoxamine scheduled daily. Therefore, more concern is lent to the interaction between fluvoxamine and apixaban. In this case, apixaban was likely indicated for treatment of non-valvular atrial fibrillation or treatment of a deep venous thrombosis based on a dose of 5mg by mouth twice daily.
Apixaban is metabolized by the liver via CYP450 to inactive metabolites. It is mainly metabolized by 3A4, however 1A92, 2C8, 2C9, 2C19 and 2J2 play minor roles in metabolism as well.1 Ibuprofen is also metabolized via CYP450 in the liver to inactive metabolites.6 When given concomitantly, both medications will be competing for metabolism by these hepatic enzymes. This will result in an increased AUC for both medications due to the liver's decreased ability to convert each drug to their respective inactive metabolites. Ibuprofen has adverse effects of gastrointestinal bleeding, inhibiting platelet aggregation and prolonging bleeding time. Apixaban is an oral anticoagulant and can further potentiate these affects when given with other drugs that have similar bleeding risks.3 While this is not a contraindication for use, as such, concomitant dosing of ibuprofen and apixaban should be monitored closely for signs and symptoms of bleeding.7
The other major drug-to-drug interaction was between apixaban and fluvoxamine. Fluvoxamine is a selective serotonin reuptake inhibitor (SSRI) used to treat obsessive-compulsive disorder (OCD).5 Fluvoxamine is a potent inhibitor of CYP1A2 and CYP2C19, a moderate inhibitor of CYP3A4, and a weak inhibitor of CYP2C9. Therefore, since fluvoxamine moderately inhibits CYP3A4 and apixaban relies on CYP3A4 in the liver to be metabolized, apixaban will remain in its active form longer instead of being metabolized to inactive metabolites. By remaining in the active form for longer than anticipated can result in increased serum concentrations leading to an increased risk of bleeding. Therefore, monitoring for signs and symptoms of bleeding is extremely important.
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The majority of Apixaban's interactions occur with drugs that have CYP interactions or drugs that increase bleeding risk. While fluvoxamine and ibuprofen do not require immediate dose reductions, it is critical to monitor for signs and symptoms of bleeding. If the above patient had presented with unexplained blood in their urine, a dose reduction or even discontinuation of Apixaban would be necessary. Since the patient is currently taking Apixaban 5 mg BID, a dose reduction to 2.5 mg BID would be appropriate.14 Discontinuation of Apixaban may be necessary if persistent bleeding occurs, if the patient has a serum creatinine >1.5 mg/dL, or if the patient is >80 years old."
Mechanism of Action
Apixaban is a selective inhibitor of factor Xa in the coagulation cascade (Figure 1). It works by directly inhibiting factor Xa which is necessary for the conversion of prothrombin to thrombin. Active thrombin is responsible for fibrin clot formation, and therefore inhibition of factor Xa inhibits conversion to thrombin and ultimately blocks fibrin clot formation. Apixaban is able to inhibit both free and clot bound factor Xa and does not require antithrombin III for activity. While there is no direct effect on platelet aggregation, it can decrease aggregation due to the effect on thrombin's conversion, because thrombin is responsible for platelet aggregation induction.2
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Chemical Properties
Apixaban is an organic, heterocyclic compound with a phenylpiperidine skeleton. It has the molecular formula, C25H25N5O4, weighing 459.5 g/mol. There are five hydrogen bond acceptors and one hydrogen bond donor in apixaban. With apixaban's cLogP being 1.267, it follows Lipinski's Rule of Five, cLogP less than 5, molecular weight less than 500 Daltons, less than 5 hydrogen bond donors and less than 10 hydrogen bond acceptors. There are five rotatable bonds and zero stereocenters. Apixaban's aqueous solubility across a physiologic pH range (pH 1.2-6.8) is approximately 40-50 mcg/mL. Its strongest acid pKa is 13.12, and its strongest basic pKa is -1.6. At physiologic pH, apixaban does not ionize, therefore it is neutral.2 "
Pharmacodynamics
Apixaban's effect on clotting results from the inhibition of factor Xa. It prolongs clotting tests, specifically prothrombin time (PT), international normalized ratio (INR), and activated partial thromboplastin time (aPTT). While apixaban prolongs clotting tests, the changes to the tests themselves are small and variable and therefore not used to monitor anticoagulation in patients. In studies, apixaban's effect on factor Xa activity was measured by the Rotachrom Heparin chromogenic assay, but this test is also not used in patients.4 Apixaban has high selectivity and potency for free and prothrombinase bound factor Xa, with a Ki for factor Xa of 0.8 nM.9,11,12 Additionally, apixaban has very low selectivity for other clotting factors and natural anticoagulant molecules, with Ki values above 15,000 nM for most and above 40,000 nM for tissue plasminogen activator.12
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Pharmacokinetics
Absorption
Apixaban shows linear pharmacokinetics for doses up to 10 mg, and absolute bioavailability of approximately 50%.4 When taken orally its absorption is not affected by food.2 Apixaban reaches its maximal concentration, Cmax, around 3-4 hours after oral administration and is primarily absorbed in the small intestine and ascending colon.
Distribution
Apixaban has a low volume of distribution, 0.31 L/kg, suggesting it stays in the main blood compartment where factor Xa is found.12 It is moderately protein bound at 87% in plasma."
Metabolism
Unchanged apixaban is the main circulating molecule in plasma.4,10 Apixaban is primarily metabolized by Phase 1 reactions, o-demethylation and hydroxylation.13 An o-demethylation metabolite (M2) can be formed at the 3-oxopiperidinyl moiety of apixaban. M2 can then form a sulfate metabolite (M1). Hydroxylation of apixaban leads to formation of 30-hydroxy apixaban (M7).13 These three, M1, M2 and M7, metabolites are the most abundant of apixaban's circulating metabolites, but all are inactive factor Xa inhibitors.13 CYP3A4 is the major metabolizer of apixaban, with CYP1A2, 2C8, 2C9, and 2J2 having minor contributions.2,4
Elimination
Apixaban has a total clearance of approximately 3.3L/h. Apixaban's dose is eliminated by hepatic metabolism, renal excretion, and gastrointestinal/bile secretion, with each being responsible for approximately one-third of a dose.10,12
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Apixaban has had many different names throughout the drug's inception ranging from the brand name Eliquis to the chemical name, 1-(4-Methoxyphenyl)-7- oxo-6-(4-(2-oxopiperidin-1-yl)phenyl)- 4,5,6,7-tetrahydro-1H-pyrazolo[3,4- c]pyridine-3-carboxamide. However, the first name used was Compound 40.9
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Compound 40 was the endpoint of a long lineage of proposed oral factor Xa inhibitors. As would be expected, there would not be a Compound 40 without a Compound 1. Compound 1 was considered a breakthrough in oral factor Xa inhibitors for 2 main reasons: the pyrazole scaffold and the 5-carboxamido linker that connects the scaffold to the P4 moiety.9 The pyrazole scaffold of Compound 1 was found to have a large impact on the potency to which the molecule binds factor Xa. The 5-carboxamido linker is important from a safety and side effects standpoint. This is because a mutagenic aniline fragment could be liberated if the 5- carboxamido linker was susceptible to metabolic hydrolysis"
The pyrazole scaffold and the carboxamido linker were the bare bones of what would eventually become Compound 40. From Compound 1 through Compound 40, there were many different optimization strategies that tinkered with the scaffold and linker group to increase efficacy and the overall pharmacokinetic profile of the drug. The first strategy was to ensure the carboxamido linker was not hydrolyzed to the mutagenic aniline moeity. This was achieved by cyclizing the linker, forming a bicyclic pyrazole. Fortunately, the bicyclic pyrazole improved the stability of the linker but also increased factor Xa potency.
The next optimization strategy looked at substituting R groups at the C3 position.9 This is particularly important because this region was found to bind the active site of factor Xa. Substituting polar groups at the C-3 position was thought to increase the bioavailability and overall pharmacokinetic profile of the drug. From these substitutions arose Compound 13f which utilized a carboxamido group at the C3 position which further increased potency. This compound also exhibited excellent pharmacokinetic parameters in dog models including high oral bioavailability (F = 100%) low clearance (Cl = 0.32 L kg-1 h-1), moderate volume of distribution (Vdss = 1.6 L kg-1), and a half-life of 5.6 h. 9
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The final optimization strategy that eventually led to the development of Compound 40 was the optimization of the P4 moiety. One particular compound stood out from all of the others, Compound 36d. The use of nitrogen containing P4 compounds had been relatively unsuccessful except for Compound 36d which utilized an N-methylacetyl group tethered to a phenyl group in the P4 position. This compound had a much higher potency than the previous nitrogen containing P4 compounds. It was found that this was due to the orientation of the N-methyacetyl group in the factor Xa active site. A closer look at the model of 36d showed the N-methyl group forming a lipophilic pi interaction with the Trp215 residue of factor Xa. This interaction allowed the acetyl carbonyl group to be aligned perpendicular to the phenyl group it was tethered to. The perpendicular orientation allowed additional hydrophobic interactions between the acetyl carbonyl group and other residues near the Trp215 residue. This was confirmed by compounds 36e and 37 which had a more planar configuration and therefore had decreased affinity for factor Xa.9 "
Compound 40 was essentially formed by combining the best parts of Compounds 5, 13f and 36d. Compound 36d was fairly lipophilic and had high protein binding. However, this was effectively counteracted by reintroducing the polar C-3 carboxamido moeity from Compound 13f into Compound 36d. This addition to compound 36d resulted in the discovery of Compound 40 which had increased potency and affinity for factor Xa due to the cyclization of the N-methylacetyl group at P4 into a gamma-lactam group (discovered in Compound 36d). Compound 40's pharmacokinetic profile was further optimized by adding the C-3 carboxamido moiety (discovered in Compound 13f) and had a better safety profile due to the bicyclic pyrazole group (discovered in Compound 5.) "
Apixaban (GG2) binding Factor Xa"Apixaban, an oral reversible direct factor Xa inhibitor is mainly metabolized by CYP3A4 to an inactive metabolite. Concomitant use of apixaban with CYP3A4 inducers or inhibitors may result in drug to drug interactions. Common drug classes that interact with apixaban include anti-platelets (NSAIDs), anticoagulants, and other drugs that have effects on the CYP enzyme system. The case study revealed an interaction between apixaban and the selective serotonin reuptake inhibitor (SSRI), fluvoxamine. The interaction between these two medications is due to the SSRI inhibiting CYP enzymes needed for apixaban's metabolism. Fluvoxamine is a moderate CYP3A4 inhibitor. Fluvoxamine also inhibits other CYP enzymes that to a lesser extent metabolize apixaban. Inhibition of CYP3A4 by fluvoxamine causes the concentration of apixaban in its active form to increase in the body. With more active apixaban in the body for a prolonged period of time, there is an increased risk of bleeding. This is especially dangerous because there is no known reversal agent for apixaban. Thus a patient with high serum concentrations of apixaban may suffer from excessive bleeding, which may be fatal. Unlike conventional anticoagulants, novel anticoagulants like apixaban do not require routine monitoring. For instance, with the anticoagulant warfarin, routine monitoring of a patient's International Normalized Ratio (INR) is needed. If the INR is high, medical intervention can be sought before a major bleed occurs. Warfarin has a reversal agent, which is vitamin K. The overall result of this fluvoxamine/apixaban interaction is increased risk of bleeding that may go undetected due to no required monitoring.
Despite the lack of reversal agent, apixaban can be used successfully in patients. Apixaban is an important oral secondary option. Patients that cannot physically tolerate the limited number of other available anticoagulants or patients who are unable to commit to an
intense monitoring regimen that is required for warfarin therapy are good candidates for apixaban therapy. Apixaban's oral bioavailability is a major reason for its success. Its bioavailability is due to the drug's small size (<500 Daltons), aqueous solubility (40-50 mcg/mL), and neutral charge at physiological pH. This neutral charge is important so that apixaban can be absorbed across biological membranes aiding its entrance into the blood stream where apixaban inhibits factor Xa. Apixaban also has a low volume of distribution, meaning most of the drug stays in the blood to inhibit factor Xa.
Apixaban's elimination is approximately evenly split three ways between hepatic, renal, and GI tract elimination pathways. This is somewhat advantageous because damage to only one of these systems (i.e. hepatic cirrhosis) wont have as great of an effect on metabolism of drug since the other two mechanisms of metabolism are still functional. However, damage to multiple drug eliminating organs (i.e. chronic kidney disease and hepatic cirrhosis) could potentially increase or decrease the amounts of active drug and inactive metabolites in the body. These patients would especially be at risk for the inability to eliminate apixaban. Poor elimination leads to increased concentrations in the body, which leads to an increased risk of bleeding."
As a result of these findings, patients on apixaban with or without interacting medications (like our patient on ibuprofen and fluvoxamine) should be counseled of sign and symptoms of bleeding. This could include blood in the spit, blood in the urine or stools, excessive nosebleeds and excessive bruising. It is also important that we monitor the patient closely as there is no reversal agent for apixaban if an excessive bleeding event were to occur. If a patient has any signs or symptoms of bleeding they should seek immediate medical assistance. Time is of the essence because these symptoms can persist for 24 hours after the last dose of apixaban.4 As stated earlier in the discussion, there is no known reversal agent for apixaban. However, activated charcoal may be beneficial in reducing the absorption of apixaban, but it must be administered 2 to 6 hours after ingestion.4
While novel anticoagulant drugs are convenient to use because of their non-requirement for routine monitoring, it is imperative to have reversal agents for these drugs to treat cases of overdose or emergent surgery, which may result in excessive hemorrhage and ultimately death. Currently, there is scanty information or data available about reversal agents for new anticoagulant drugs. In fact, there are no known reversal agents for direct factor Xa (FXa) inhibitors like apixaban. Furthermore, there is no conclusive evidence to support the efficacy of available reversal agents for other anticoagulants including activated prothrombin complex concentrate (aPCC), three-factor prothrombin complex concentrates (PCCs), and recombinant factor VIIa (rFVIIa) in treating hemorrhagic events or overdose cases related to apixaban.8 Therefore, there is need for aggressive research for a reversal agent for apixaban and other direct FXa inhibitors because of their therapeutic superiority compared to classic drugs like warfarin. The future of anticoagulant therapy is venturing down the path of one-size-fits-all, direct action, and no monitoring required therapy. Research and development of a reversal agent for apixaban is imperative.
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A sincere thank you to the following preceptors for contributing to our discussions and development of our project and pharmacy knowledge: Dr. Daniel Sem PhD, Concordia University of Wisconsin, Mequon WI; Mini Vijayan PharmD, Walgreens, Thiensville WI."
Thank you to Katherine Balcer, Elizabeth Pegelow, and Morgan Steffens for creating the content for the paper poster; Nicole Endres for designing and compiling the information onto the paper poster; Sydney Bishop, Kyle Weaver and Ruth Heideman for creating all of the jmol images and applying them to the eposter; Daniel Wilk and Patricia Wirnkar for editing and compiling the textual information on the eposter. Also, Katherine Balcer, Sydney Bishop, Nicole Endres, Ruth Heideman, Patricia Wirknar, Elizabeth Pegelow, Morgan Steffens, Kyle Weaver, and Daniel Wilk for help with editing."
1) Apixaban. In: Clinical Pharmacology [database online]. Elsevier/Gold Standard; 2014. http://0-www.clinicalpharmacology-ip.com.topcat.switchinc.org/Forms/Monograph/monograph.aspx?cpnum=3795&sec=monphar&t=0. Updated October 22, 2014. Accessed November 13, 2014.
2) Apixaban. In: DrugBank [database online]. AB, Canada: The Metabolomics Innovation Centre; 2008. http://www.drugbank.ca/drugs/DB06605. Updated September 16, 2013. Accessed November 7, 2014.
3) Drug Interaction Report. In: Clinical Pharmacology [database online]. Elsevier/Gold Standard; 2014. http://0-www.clinicalpharmacology-ip.com.topcat.switchinc.org/Forms/Reports/intereport.aspx?cpnum=303,3795&l=0. Accessed November 13, 2014.
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4) Eliquis (apixaban). [package insert]. Princeton, New Jersey: Bristol-Myers Squibb Company; 2014.
5) Fluvoxamine. In: Clinical Pharmacology [database online]. Elsevier/Gold Standard; 2014. http://0-www.clinicalpharmacologyip.com.topcat.switchinc.org/Forms
/Monograph/monograph.aspx?cpnum=265&sec=mondesc&t=0. Updated July 23, 2014. Accessed November 12, 2014.
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6) Ibuprofen. In: Clinical Pharmacology [database online]. Elsevier/Gold Standard; 2014. http://0-www.clinicalpharmacology-ip.com.topcat.switchinc.org/Forms/Monograph/monograph.aspx?cpnum=303&sec=mondesc&t=0. Updated January 9, 2014. Accessed November 13, 2014.
7) Interactions. In: Lexicomp [database online]. St. Louis, MO: Wolters Kluwer Health, Inc; 2014. http://0-online.lexi.com.topcat.switchinc.org/lco/action/interact. Accessed November 13, 2014.
8) Miyares MA, Davis K. Newer oral anticoagulants: a review of laboratory monitoring options and reversal agents in the hemorrhagic patient. Am J Health Syst Pharm. 2012;69(17):1473-84.
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9) Pinto D, Orwat M, Koch S, et al. Discovery of 1-(4-Methoxyphenyl)-7-oxo-6-(4-(2-oxopiperidin-1-yl)phenyl)-4,5,6,7-tetrahydro-1H-pyrazolo[3,4-c]pyridine-3-carboxamide (Apixaban, BMS-562247), a Highly Potent, Selective, Efficacious, and Orally Bioavailable Inhibitor of Blood Coagulation Factor Xa. J. Med. Chem. 2007; 50(22):5339-56.
10) Raghavan N, Frost CE, Yu Z, et al. Apixaban Metabolism and Pharmacokinetics after Oral Administration to Humans. Drug Metabolism and Disposition. 2008; 37(1):74-81. http://dmd.aspetjournals.org/content/37/1/74.full. October 2, 2008. Accessed November 7, 2014.
11) Turpie Alexander G.G. Oral, Direct Factor Xa Inhibitors in Development for the Prevention and Treatment of Thromboembolic Diseases. Ateriosclerosis, Thrombosis, and Vascular Biology Journal of the AHA. 2007; 27: 1238-1247. http://atvb.ahajournals.org/content/27/6/1238.full. March 22, 2007. Accessed November 7, 2014.
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12) Wong PC, Pinto DJP, Zhang D. Preclinical discovery of apixaban, a direct and orally bioavailable factor Xa inhibitor. Journal of Thrombosis and Thrombolysis. 2011;31(4):478-492. http://www.ncbi.nlm.nih.gov/pubmed/21318583. May 2011. Accessed November 7, 2014.
13) Zhang D, He K, Raghavan N, Wang L, et al. Comparative Metabolism of 14C-Labeled Apixaban in Mice, Rats, Rabbits, Dogs, and Humans. Drug Metabolism and Disposition. 2009; 37(8)1738-1748. http://dmd.aspetjournals.org/content/37/8/1738.long. August 2009. Accessed November 7, 2014.
14) Zalpour A, Oo TH. Clinical utility of apixiban in the prevention and treatment of venous thromboembolism: current evidence. Drug Design, Development and Therapy. November 2014. Accessed March 23, 2015."
15.) Thrombosis Advisor. Coagulation cascade animation - physiology of hemostasis. YouTube. March 3, 2014. https://www.youtube.com/watch?v=cy3a__OOa2M"