CfP1 2020 Integrated Assignment

1). Examining the data given below for WARFARIN, examine and explain how the key physiochemical properties can be related to the chemical structure and structural features of warfarin.

(25 marks)

Warfarin
Enoxaparin

pKa
5
–2, –2, –2, 2.9 per repeating unit

MW
310
~5000 (typically ~16 repeating units)

Log P
3.4
< –4 per repeating unit Log D7.4 1 < –8 per repeating unit HBD’s 1 >20 in total

Stereoisomerism
Enantiomers
Many stereocentres

[WRITE YOUR ANSWER HERE]

The compound is derived from plant-based molecules and has a functional hydroxyl group attached to the central ring.

Figure 1. Warfarin structure

The Warfarin molecule is very stable because of the resonance that occurs at an intramolecular level because of the sp2 hybridizing of its carbon atoms. During ionization stage, Warfarin loses a hydrogen ion from the hydroxyl group to form a conjugate base of the compound.

Figure 2. Formation of conjugate base of Warfarin

The hydroxyl group, when ionized, forms a lone pair of electrons which are resonated over five centres within the ring making the molecule highly stable. This resonance improves solubility of the molecule because at physiological pH, it is almost completely ionized.

Figure 3. Resonance of negative charge

The compound is similar to carboxylic acid in that it has pKa value of 5 indicating that it is weakly acidic. The pKa of the compound is established by the hydroxyl group in its ability to lose a hydrogen ion as acids do.

Warfarin works as a Vitamin K antagonist and is a racemic mixture of two isomers, R and S (Jeske, 2019).The molecule has a chiral carbon centre that allows for flexibility thus the display of stereoisomerism.

Figure 5: Stereoisomers of warfarin (5

The chiral carbon enables the molecule to bind to receptors or plasma protein. The drug can form pi-pi interactions via hydrogen bonding and Van der Waals forces of attraction. With a physiological pH of 7.4 and a Log D value of 1, Warfarin is distributed effectively in both aqueous and fatty phases.

2). Explain how the structure and molecular properties of WARFARIN determines and influences its pharmacokinetic properties such as solubility, permeability, clearance, plasma protein binding and volume of distribution as displayed below.

(30 marks)

Warfarin
Enoxaparin

Solubility
Very high
Very high

Permeability
Moderate
Very low

Clearance
Low
‘Natural rate’

Plasma Protein binding
99%
80%

Volume of distribution
8L (low)
4L (low)

[WRITE YOUR ANSWER HERE]

There are three factors that contribute to the high solubility of the Warfarin molecule: resonance that leads to formation of a stable conjugate base, polarity of oxygen atoms, and high ionization levels. At physiological pH of 7.4, Warfarin is almost completely ionized and dissolves equally in both fatty and aqueous phases. High level of ionization allows for hydrogen bonds to be formed between the drug and water molecules. This can be confirmed using the general solubility equation as indicated below:

logS = 0.5 – 0.01 (MP – 25) –logP

         = 0.5 – 0.01 (162* – 25) – 3.4 

         = -4.27

The absorption of Warfarin in the gastrointestinal tract occurs rapidly and almost completely. In the stomach, the drug exists predominantly in unionized form. Since the Log P of the drug is 3.4, the unionized version is 3.4 times lipophilic than it is hydrophilic. Consequently, when Warfarin is in an aqueous environment, it is more likely to be bound to plasma proteins such as Albumin when it leaves the small intestines and enters the hepatic portal blood.

Warfarin has moderate permeability. As it traverses the cell membrane lipid barrier, it exists in unionized and unbound state. Since the bound and unbound states of Warfarin are in equilibrium in the small intestines, the drug will be able to permeate the barrier. However, the lipophilic portions of the drug molecules do not entirely pass through the lipid bilayer and remain bound within the walls of the small intestines. Warfarin has low clearance levels because it has up to 99% protein binding in plasma. The clearance is mainly renal but some studies have indicated an interaction between Warfarin and the ABCB1 transporter found in the liver.

3) For WARFARIN and ENOXAPARIN, explain and compare how the above physiochemical and pharmacokinetic properties impact on the administration, clinical use and effects of these drugs. Please include an explanation of any differences in usage during pregnancy and how these relate to the drug’s properties (10 marks).

(40 marks in total)

[WRITE YOUR ANSWER HERE]

Various proteins and enzymes are involved in the thrombus formation process.

Figure 6. Thrombus formation process

Targeting a specific component of the above process will disrupt thrombus formation hence have an anticoagulation effect. The target of action by Warfarin is inhibition of the vitamin K epoxide reductase (VKOR) complex 1 (PharmGKB, 2020). To this end, S-Warfarin is upto 5 times more potent than R-Warfirin. VKOR inhibition interferes with activation of Factor Xa precursors and as a result, prothrombin is not converted to thrombin. Warfarin depletes functional vitamin K reserves reducing synthesis of active clotting factors (Patel, Singh, Preuss, & Patel, 2019).

Enoaxaparin, on the other hand, works by increasing antithrombin levels thus inhibiting the conversion of pro-thrombin to thrombin. It prevents the propagation and growth of existing thrombi but does not breakdown existing clots (Nutescu, Burnett, Fanikos, Spinler, & Wittkowsky, 2016). The drug is specifically active against factor Xa and thrombin (Willihnganz, Gurevitz, & Clayton, 2019).

The enoxaparin repeating unit has four functional groups: one carboxylate and three sulphate groups as shown below:

Figure 7. Enoxaparin

Enoxaparin is a complex mixture of oligosaccharides varying in structure and size (U.S. FDA, 2018). Its chemical properties are determined by heparin properties and the chemical processes undertaken in breaking the heparin into short chains of the enoxaparin. The drug has a logD value less than -8 thus it is hydrophilic when ionized at physiological pH. Made from heparin, enoxaparin lasts longer in the body thus it is administered subcutaneously (U.S. FDA, 2018).

Warfarin and Enoxaparin differ in their ionization mechanisms. While Warfarin can donate only one hydrogen bond, Enoxaparin has the capacity to donate three hydrogen bonds for every repeating unit.

Warfarin takes between 24 and 72 hours after administration for its action to start and peak therapeutic effect 5 to 7 days after initiation. However, it has two main benefits: being rapidly and completely absorbed, and 99% protein binding (Patel, Singh, Preuss, & Patel, 2019). For these two reasons, it is appropriate for oral administration. Additionally, 92% of Warfarin is excreted in urine through glomerular filtration in the kidney because its aromatic rings provide numerous sites for aromatic hydroxylation to occur.

Both of these drugs have contraindications that should be considered before prescription. The high absorption rate of Warfarin is disadvantageous to pregnant women because it traverses the placental barrier causing fetal plasma levels to rise to be the same with maternal levels (Patel, Singh, Preuss, & Patel, 2019). When used in the first trimester, it is likely to cause spontaneous abortion in a condition referred to as Fetal Warfarin Syndrome (FWS). Warfarin also has the associated risk of bleeding and significant haemorrhage dependent on patient susceptibility and intensity of anticoagulation. The risk is especially significant after high-risk procedures like polypectomy and remains high for several days (Lee, Ross, Rivadeneira, Steele, & Feingold, 2017).

Enoxaparin does not cross the placental barrier because it is a bulky molecule, thus is used instead of Warfarin in pregnancy. Even so, if the woman has renal impairment, specific advice should be given because Enoxaparin exits the body through renal excretion (Nottinghamshire Area Prescribing Committee, 2019). If the woman has normal renal functioning, the dosage is based on early pregnancy weight.

References

Jeske AH. Contemporary Dental Pharmacology: Evidence-Based Considerations. Berlin, Heidelberg: Springer; 2019.

Lee SW, Ross HM, Rivadeneira DE, Steele SR, Feingold DL. Advanced Colonoscopy and Endoluminal Surgery. Berlin, Heidelberg: Springer. 2017.

Nottinghamshire Area Prescribing Committee.  Information sheet for Enoxaparin for Primary Care Prescribers [Internet]. NHS Trust . NHS Trust ; 2019 [cited 2020 March 18]. Available from: https://www.nottsapc.nhs.uk/media/1080/enoxaparin-info-sheet.pdf

Nutescu EA, Burnett A, Fanikos J, Spinler S, Wittkowsky A. Pharmacology of anticoagulants used in the treatment of venous thromboembolism. Journal of Thrombosis and Thrombolysis. 2016 Jan;41(1):15–31.

Patel S, Singh R, Preuss CV, Patel N. Warfarin [Internet]. StatPearls Publishing LLC; 2019 [cited 2020 Mar 17]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470313/

PharmGKB. Warfarin [Internet]. PharmGKB; 2020 [cited 17 Mar 2020]. Available from: https://www.pharmgkb.org/chemical/PA451906

U.S. FDA. Generic Enoxaparin Questions and Answers [Internet]. U.S. FDA; 2018 [cited 2020 Mar 18]. Available from: https://www.fda.gov/drugs/postmarket-drug-safety-information-patients-and-providers/generic-enoxaparin-questions-and-answers

Willihnganz M, Gurevitz SL, Clayton BD. Clayton’s Basic Pharmacology for Nurses. Amsterdam: Elsevier Health Sciences; 2019.