758
P A R T 8
Drugs acting on the cardiovascular system
A
ntiplatelet agents
Antiplatelet agents decrease the formation of the
platelet plug by decreasing the responsiveness of the
platelets to stimuli that would cause them to stick and
aggregate on a vessel wall. Antiplatelet agents avail
able for use include abciximab (
ReoPro
), anagrelide
(
Agrylin
),
aspirin, cilostazol (
Pletal
[not available in
New Zealand]), clopidogrel (
Plavix
), dipyridamole
(
Persantin
), eptifibatide (
Integrilin
), prasugrel (
Effient
),
ticagrelor (
Brilinta
), ticlopidine (
Tilodene
), and
tirofiban (
Aggrastat
).
Therapeutic actions and indications
The antiplatelet agents inhibit platelet adhesion and
aggregation by blocking receptor sites on the platelet
membrane, preventing platelet–platelet interaction or
the interaction of platelets with other clotting chemicals.
One drug, anagrelide, blocks the production of platelets
in the bone marrow. These agents are used effectively to
treat cardiovascular diseases that are prone to produce
occluded vessels; for the maintenance of venous and
arterial grafts; to prevent cerebrovascular occlusion;
and as adjuncts to thrombolytic therapy in the treat
ment of myocardial infarction (MI) and the prevention
of reinfarction after MI. The prescriber’s choice of drug
depends on the intended use and the person’s tolerance
of the associated adverse effects. See Table 48.1 for usual
indications for each of these agents.
Pharmacokinetics
Abciximab, eptifibatide and tirofiban are adminis
tered intravenously (IV). Antiplatelet agents that are
administered orally include anagrelide, aspirin, cilosta
zol, clopidogrel, prasugrel, ticagrelor and ticlopidine.
Dipyridamole is used orally or as an IV agent.
These drugs are generally well absorbed and highly
bound to plasma proteins. They are metabolised in the
liver and excreted in urine, and they tend to enter breast
milk (see Contraindications and cautions).
Contraindications and cautions
Antiplatelet agents are contraindicated in the presence
of allergy to the specific drug
to avoid hypersensitivity
reactions
. Caution should be used in the following con
ditions: the presence of any known bleeding disorder
because of the risk of excessive blood loss
; recent surgery
because of the risk of increased bleeding in unhealed
vessels
; and closed head injuries
because of the risk of
bleeding from the injured vessels in the brain.
Although there are no adequate studies of these drugs
in pregnancy, they are contraindicated
because of the
potential for increased bleeding
(see Adverse effects); they
should be used during pregnancy only if the benefits to the
mother clearly outweigh the potential risks to the fetus.
These drugs are also contraindicated during breastfeed
ing
because of the potential adverse effects on the fetus or
neonate
; if they are needed by a breastfeeding woman, she
should find another method of feeding the baby.
Anagrelide should be used with caution with any
history of thrombocytopenia
because it decreases the
production of platelets in the bone marrow
. Platelet
levels should be checked regularly to monitor for throm
bocytopenia if a person is on this drug.
Adverse effects
The most common adverse effect seen with these drugs
is bleeding, which often occurs as increased bruising
and bleeding while brushing the teeth. Other common
problems include headache, dizziness and weakness;
the cause of these reactions is not understood. Nausea
TABLE 48.1
DRUGS IN FOCUS Drugs affecting clot formation and resolution (continued)
Drug name
Dosage/route
Usual indications
Anticoagulant adjunctive therapy (continued)
vitamin K (Konakion Adult,
Konakion MM)
Konakion Adult: Adult: 2.5–10 mg PO or IV
Konakion MM: Neonatal prophylaxis: 2 mg PO
at birth, repeated at 3–5 days and 4 weeks,
or 1 mg IM at birth
Treatment of anticoagulant-induced
prothrombin deficiency
Haemorheologic agent
oxypentifylline (Trental)
400 mg PO t.d.s. with meals
Treatment of intermittent claudication to
improve function and reduce symptoms;
improve blood flow in vascular diseases
Many herbal therapies can cause problems when used
with drugs that affect blood coagulation. People taking
these drugs should be cautioned to avoid angelica, cat’s
claw, chamomile, chondroitin, feverfew, garlic, ginkgo,
goldenseal, grape seed extract, green leaf tea, horse
chestnut seed, psyllium and turmeric. If a person who is
taking an anticoagulant presents with increased bleeding
and no other interaction or cause is found, question the
person about the possibility of use of herbal therapies.
Herbal and alternative therapies
BOX 48.2
