Parnaparin

Summary

Synopsis

Parnaparin is a low molecular weight (LMW) heparin which, like other members of its class, apparently demonstrates a greater antithrombotic effect relative to its anticoagulant activity when compared with the unfractionated heparin (heparin) from which it is derived. Moreover, subcutaneous parnaparin has a greater bioavailability and longer half-life than heparin, permitting once-daily administration for the prophylaxis of deep venous thrombosis (DVT) or the treatment of established vascular disorders

Prophylaxis with a 7-day regimen of parnaparin 3200 or 6400 IUaXa/day has consistently been associated with a lower incidence of confirmed DVT compared with usual prophylactic regimens of heparin. This intertreatment difference reached statistical significance in a large multicentre study involving a total of 610 surgical patients (3.2% for parnaparin vs 6.3% for heparin). Thus far, however, comparisons of parnaparin with other LMW heparins for this indication are unavailable.

Parnaparin has demonstrated equivalent efficacy to heparin in the treatment of established vascular disorders, including phlebopathies and related syndromes, as well as peripheral arterial occlusive disease. Parnaparin also showed some benefit as an adjunctive therapy in patients with angina pectoris.

The risk of general bleeding appears to be similar with parnaparin or heparin, although parnaparin results in fewer haematomas at the site of injection, partly because of the less frequent administration regimen. Parnaparin has also been associated with a lower incidence of pain and/or burning sensation at the injection site compared with heparin.

As with other LMW heparins, the possibility that parnaparin will be infrequently associated with thrombocytopenia cannot be excluded.

Thus, parnaparin may be preferred over traditional heparin for the prophylaxis of thrombo-embolic events in surgical patients (particularly those at high risk for DVT), as well as the treatment of established vascular disorders with a thrombotic aetiology. Compared with heparin, parnaparin offers the advantages of a more convenient administration regimen coupled with improved local tolerability. However, the therapeutic advantages of parnaparin relative to other LMW heparins have yet to be established in large scale comparative trials.

Pharmacodynamic Properties

Parnaparin is a low molecular weight (LMW) heparin with a mean molecular weight of 4.5kD. Like other members of its class, parnaparin has been developed because it apparently demonstrates a greater antithrombotic effect relative to its anticoagulant activity when compared with the unfractionated heparin (heparin) from which it is derived

In rodents, parnaparin has demonstrated antithrombotic activity equivalent or superior to that of heparin and a lesser prohaemorrhagic effect.

Tested doses of parnaparin ≤12 800 IUaXa produce dose-dependent inhibition of factor Xa, although there is some interstudy variation in the duration of action. In fact, significant inhibition of factor Xa activity has lasted for up to 24 hours following subcutaneous administration of parnaparin 6400 IUaXa in healthy volunteers. In a comparative study, parnaparin 6400 IUaXa produced inhibition of factor Xa that was both greater and longer lasting than that following heparin 10 000IU. At the same time, parnaparin had a lesser antithrombin effect than heparin, with a smaller increase in the activated partial thromboplastin time (aPTT). The peak increase in aPTT with parnaparin appears to be of minimal clinical significance, even at the highest dose tested (12 800 IUaXa).

Parnaparin is significantly less potent than heparin in potentiating human platelet aggregation induced with agonists, and retains a significantly higher anticoagulant activity in the presence of activated platelets.

The in vitro aPTT activity of parnaparin could be fully neutralised with protamine salts, although the anti-factor Xa (anti-Xa) activity was only partially neutralised, even at the highest protamine dose tested (100 αg/L).

Pharmacokinetic Properties

As with other LMW heparins, the pharmacokinetic properties of parnaparin have been determined indirectly by measurement of anti-Xa activity in the plasma

Compared with subcutaneous administration, the maximal anti-Xa activity is approximately 5-fold higher following intravenous administration of parnaparin. Nonetheless, the subcutaneous bioavailability of parnaparin is estimated to be more than 90%; maximal anti-Xa activity is reached 2 to 4 hours post-injection. Parnaparin bioavailability is independent of the site of subcutaneous administration and similar to that for the intramuscular route.

During repeated subcutaneous administration, a constant level of anti-Xa activity is reached within 2 to 4 days. Once-daily administration results in minimal accumulation of parnaparin beyond the first day, even at the highest dose tested (12 800 IUaXa). However, the likelihood of a cumulative effect may be increased with a twice-daily regimen of parnaparin.

Parnaparin, in common with other LMW heparins, is eliminated primarily by a nonsaturable renal mechanism. Following intravenous administration, plasma clearance was around 0.03 L/h/kg, independent of the tested dose. In contrast, the plasma half-life increased dose-dependently, reaching 134 minutes at the highest dose evaluated (3200 IUaXa).

The pharmacokinetic profile of parnaparin in patients with hepatic or renal failure has yet to be evaluated.

Clinical Efficacy

In clinical trials, subcutaneous administration of parnaparin has proven effective for the prevention of postsurgical deep venous thrombosis (DVT) and pulmonary embolism. Typically, patients at relatively low risk for DVT have received parnaparin 3200 IUaXa/day, while patients at high risk for DVT have received parnaparin 6400 IUaXa/day administered once or twice daily. Parnaparin 6400 IUaXa/day is also effective in maintaining the patency of prosthetic conduits used during arterial repair

In comparative studies, the incidence of confirmed DVT with parnaparin tended to be lower than with standard prophylactic regimens of heparin. In the largest multicentre study, DVT developed in significantly fewer patients receiving parnaparin 3200 or 6400 IUaXa/day compared with patients receiving heparin 10 000 or 15000 IUaXa/day [3.2% (n = 308) vs 6.3% (n = 302)]. In this study, the incidence of pulmonary embolism was also lower with parnaparin treatment (0.3 vs 1 %), although this difference did not reach statistical significance.

Parnaparin, typically administered at a maintenance dosage of 4250 or 6400 IUaXa/day, significantly improved objective measures of venous haemodynamics, as well as subjective assessments of symptoms in patients with various phlebopathies and related syndromes. In comparative trials, parnaparin demonstrated efficacy equivalent to that of heparin ≤25 000 IU/day. Parnaparin 4250 IUaXa/day was also as effective as heparin 15 000 IU/day in providing relief from symptoms associated with chronic venous insufficiency.

Compared with placebo, parnaparin 6400 IUaXa/day has significantly improved objective measures of arterial function as well as subjective assessments of clinical symptoms in patients with peripheral arterial occlusive disease. In a single study, 6 to 8 months’ treatment with parnaparin demonstrated efficacy equivalent to that of heparin 10 000 IU/day.

When investigated as an adjunct to conventional therapy, 3 months’ treatment with parnaparin 6400 IUaXa/day significantly increased the ischaemic threshold in patients with stable angina pectoris.

Tolerability

Parnaparin, in common with other LMW heparins, has the potential to cause an increase in bleeding as an extension of its antithrombotic effect. As expected, meta-analysis reveals a direct relationship between the frequency of haemorrhagic-like episodes and parnaparin dose. However, there is little evidence to suggest a marked increase in the risk of perioperative blood loss and need for transfusion with therapeutic dosages of parnaparin 3200 or 6400 IUaXa/day

The risk of perioperative haemorrhage with parnaparin appears to be similar to that for standard prophylactic regimens of heparin, although comparative trials may have been of insufficient size to reveal a statistically significant difference (if any) between treatments. In the largest trial, clinically detectable haemorrhages occurred in 1% of general surgery patients receiving parnaparin 3200 or 6400 IUaXa/day for 7 days, compared with 4% of patients receiving heparin 10 000 or 15 000 IU/day.

The local tolerability of parnaparin is generally superior to that of heparin, as evidenced by the appearance of significantly fewer haematomas at the injection site, even after correcting for the lower administration frequency with parnaparin. In addition, parnaparin has been associated with a lower incidence of pain and/or burning sensation at the injection site, although the overall incidence is low with both treatments.

Neither heparin-sensitive clotting assays (e.g. aPTT) nor common laboratory parameters are affected to a clinically significant extent by parnaparin.

Though rare, thrombocytopenia has been reported in association with several commercially available LMW heparins. Thrombocytopenia has not been reported to date in clinical trials of parnaparin, although clinical experience with the drug is, as yet, limited.

Dosage and Administration

In patients at relatively low risk of developing DVT, a single subcutaneous injection of parnaparin 3200 IUaXa is usually given 2 hours prior to surgery. Injections are then repeated every 24 hours for at least 7 days. In clinical trials, patients at higher risk of developing DVT, such as those undergoing orthopaedic surgery, have received parnaparin 6400 IUaXa/day administered in a once or twice daily regimen. The manufacturer, however, recommends that parnaparin 4250 IUaXa be administered 12 hours before and after surgery, followed by a daily injection for a minimum of 10 days in high risk patients

Patients with phlebopathies and related syndromes have typically received a maintenance dosage of parnaparin 4250 or 6400 IUaXa/day for up to 3 months, if required. In comparison, patients with peripheral vascular disease of arterial occlusive origin have received parnaparin 6400 IUaXa/day for 6 to 8 months.

Anti-Xa activity should be carefully monitored in patients with renal dysfunction, as parnaparin clearance may be reduced in these circumstances.