The following information has been extracted and summarised from various source documents, whose references have been listed below:
Aspirin is the most widely studied antiplatelet drug. On the basis of >100 randomized trials in high-risk patients, aspirin is shown to reduce vascular death by ~15%a and non-fatal vascular events by ~30%.(a)
The indirect comparisons reported in the overview of the Antithrombotic Trialists' Collaboration summarised 12 trials with a total of 6,776 high risk patients, confirmed that doses between 75-150 mg daily reduced vascular events by 26 – 38%. Doses over 160 mg reduced these odds by up to 22% according to another 53 trials (48,964 patients) while doses less than 75 mg dropped to as low as 5% evidenced by 3 trials and 3,655 patients.(b)
Equivalent doses of the enteric-coated aspirin were not as effective as plain aspirin. Lower bioavailability of these preparations and poor absorption from the higher pH environment of the small intestine may result in inadequate platelet inhibition, particularly in heavier subjects.(a)
(a)Patrono C, García Rodríguez LA, Landolfi R, Baigent C. Low-dose aspirin for the prevention of atherothrombosis. N Engl J Med. 2005; 353 (22): 2373 – 2383.
(b)Antithrombotic Trialists' Collaboration. Collaborative meta-analysis of randomised trials of antiplatelet therapy for prevention of death, myocardial infarction, and stroke in high risk patients. BMJ. 2002; 324 (7329): 71 – 86.
Complete or near-complete inhibition of platelet COX-1, and consequently reduction in platelet aggregation, can be achieved with low doses of aspirin (75-150 mg) given once daily.(c)
(C)Holbrook A1, Schulman S, Witt DM, Vandvik PO, Fish J, Kovacs MJ, Svensson PJ, Veenstra DL, Crowther M, Guyatt GH; American College of Chest Physicians. Evidence-based management of anticoagulant therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest. 2012 Feb; 141 (2 Suppl): e152S-84S. doi: 10.1378/chest: 11-2295.
The oral bioavailability of regular aspirin tablets is ~40% to ~50% over a wide range of doses. A considerably lower bioavailability has been reported for enteric-coated tablets and for sustained-release, microencapsulated preparations.(d)
(d)Cox D , Maree AO , Dooley M , Conroy R , Byrne MF , Fitzgerald DJ . Effect of enteric coating on antiplatelet activity of low-dose aspirin in healthy volunteers. Stroke. 2006; 37 ( 8 ): 2153 – 2158.
Low-dose aspirin increases by twofold the risk of upper gastrointestinal complications in the general population and enteric-coated formulations do not modify the effect.(e)
(e)de Abajo FJ , García Rodríguez LA . Risk of upper gastrointestinal bleeding and perforation associated with low-dose aspirin as plain and enteric-coated formulations . BMC Clin Pharmacol. 2001; 1:1.
No clinical benefits in terms of reduction of gastrointestinal bleeding or ulceration with enteric coating have, therefore, been successfully demonstrated, although the endoscopic studies show that potentially these benefits could exist.(f)
(f)Walkera J, Robinsona J, Stewart J and Jacob S. Does enteric-coated aspirin result in a lower incidence of gastrointestinal complications compared to normal aspirin? Interactive CardioVascular and Thoracic Surgery 6. 2007; 519–522.
Comparative pricing from pharmacies including community and corporate pharmacies around the major centres of South Africa showed that Myoprin 100 mg 30's costs less than the competitors in its category.(g)
(g)Market data from February 2016 available on request.
Abbreviated Package Insert
SCHEDULING STATUS: S2 PROPRIETARY NAME (AND DOSAGE FORM): Myoprin 100 mg Tablet COMPOSITION: Each tablet contains Aspirin 100 mg. PHARMACOLOGICAL CLASSIFICATION: A 2.7 Anti-pyretic or anti-pyretic and inflammatory analgesics. PHARMACOLOGICAL ACTION: Myoprin 100 mg inhibits platelet aggregation by inactivation of platelet cyclo-oxygenase, the enzyme that produces the cyclic endoperoxide precursor of thromboxane A2. INDICATIONS: Indications relates to inhibition of platelet aggregation: To reduce the risk of myocardial infarctions in patients with unstable angina or in patients who have previous myocardial infarction. To reduce the risk of recurrent transient ischaemia attacks or stroke in men who have had transient ischaemia of the brain due to fibrin platelet emboli. To reduce the risk of graft occlusion following aorta coronary by-pass surgery. DOSAGE AND DIRECTIONS FOR USE: ADULTS: One tablet as directed by your doctor to be taken every day, preferably at the same time each day and with meals. WARNINGS: The optimal dose for inhibition of platelet aggregation in humans is not known. Do not use this product for indications related to the inhibition of platelet aggregation unless directed by a doctor. CONTRA-INDICATIONS: Should not be administered to patients with haemorrhagic disorders, with gout, or those with an intolerance to aspirin (especially aspirin-sensitive asthmatics). Use with caution in patients with impaired renal or hepatic function. Aspirin should not be taken during the first and third trimesters of pregnancy and during lactation. Aspirin should be discontinued one week before scheduled surgical procedures. SIDE EFFECTS AND SPECIAL PRECAUTIONS: Aspirin has been implicated in Reye's syndrome, a rare but serious illness in children and teenagers with chickenpox and influenza. A doctor should be consulted before aspirin is used in such patients. Aspirin may enhance the coumarin anticoagulants, sulphonylurea, hypoglycemic agents, methotrexate, phenytoin and valproic acid. Aspirin diminishes the effects of uricosuric agents such as probenecid and sulphinpyrazone. Dizziness, erosion, ulceration, heamatemesis and melaena may occur. Some persons, especially asthmatics exhibit notable sensitivity to aspirin which may provoke various hypersensitivity reactions which may include skin eruptions, urticaria, angio-edema, rhinitis, paroxysmal bronchospasm and dyspnoea. Aspirin increases the bleeding time and in large doses may cause hypoprothrombinaemia. The most common adverse effects occurring with therapeutic doses of aspirin are gastrointestinal disturbances such as nausea, hepatotoxicity particularly in patients with juvenile arthritis and other connective tissue disorders. It should be administered with caution to patients with impaired renal function, dyspepsia, anaemia and when the patient is dehydrated. For full prescribing information refer to the package insert approved by the medicines regulatory authority.