1 Laboratoires d'Hématologie, Faculté de Pharmacie, F-34060 Montpellier et Centre Hospitalier Universitaire, F-30029 Nîmes, and 2 Service de Gynécologie et Obstétrique, Hôpital Bichat, 46 rue Henri Huchard, 75018 Paris, France
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Abstract |
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Key words: acquired resistance/oral contraceptives/protein C/third-generation/thromboembolism
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Introduction |
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The inherited deficiency of the anticoagulant response to activated protein C is a risk factor for venous thromboembolism. The congenital functional plasma phenotype termed `resistance to activated protein C' was first described in 1993 (Dahlbäck et al., 1993). Inherited resistance to activated protein C is mainly caused by a single point mutation in factor V gene, leading to the synthesis of a molecular variant: factor V Leiden (Bertina et al., 1994
). The thromboembolic risk of patients carrying this mutation is increased 30-fold by the use of oral contraceptives: in homozygotes, the risk is greater at >100-fold (Vandenbroucke et al., 1994
).
Early reports have shown that, in non-carriers of the factor V Leiden mutation who are current users of oral contraceptives, the anticoagulant response to activated protein C is reduced (Osterud et al., 1994; Henkens et al., 1995
; Olivieri et al., 1995
). This led to the concept of acquired resistance to activated protein C (abnormal phenotype but normal genotype) being induced, in this case, by oral contraceptives. More generally, reduced sensitivity for activated protein C in the absence of factor V Leiden mutation has recently been described to increase the risk of venous thrombosis (DeVisser et al., 1999
).
A new methodology for testing response to activated protein C has recently been described (Rosing et al., 1997). The authors showed that third-generation oral contraceptives induce a more important impairment of the plasma response to activated protein C than do second-generation oral contraceptives.
The comparative appraisal of the side-effects of second and third-generation oral contraceptives is still a matter of debate. This led us to perform an in-depth re-evaluation of the links between oral contraceptives, the induced modifications of the plasma response to activated protein C and the thromboembolic risk, according to the available haemostasis-related tests used to characterize the plasma response to activated protein C.
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Concerning the results of Dalbäck's assay in women taking second or third-generation oral contraceptives, in the absence of factor V Leiden mutation |
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In the initial case-control study showing that an acquired plasma resistance to activated protein C is associated with an increased thromboembolic risk (DeVisser et al., 1999), women had lower values than men, an increase in age was associated with a decrease in values and high factor VIII plasma concentrations were associated with low responses. After adjustment for sex, age and factor VIII concentrations, low values still predicted a 2.5 excess of clinical risk. Women not on oral contraceptives had slightly (but not significantly) higher values than the group of women as a whole. This let readers suspect that women on oral contraceptives had not significantly lower values than the group of women as a whole: however, specific data concerning women on oral contraceptives were not shown and no data concerning the difference between second and third-generation compounds users were given.
The same team had previously shown that thrombosis patients using oral contraceptives have lower responses to activated protein C than thrombosis patients not using oral contraceptives, and that thrombosis patients using oral contraceptives have lower results than non-thrombosis controls using oral contraceptives (Bloemenkamp et al., 1998). The percentage of `high responders', i.e. those below the 25th percentile of the plasma response to activated protein C evaluated in controls in former thrombosis patients and in healthy control women, both using oral contraceptives, was 50 and 25% respectively (Bloemenkamp et al., 1998
). This favours the hypothesis that the oral contraceptive-induced variations of response to activated protein C are more important in former thrombosis patients than in asymptomatic women (Bloemenkamp et al., 1998
) and shows that an acquired resistance to activated protein C is likely to be relevant in the pathogenesis of venous thrombosis induced by oral contraceptives. However, no data are available on the differential effects of second and third-generation drugs.
In a study comparing the performances of two different functional tests, no significant difference in sensitivity to activated protein C between users of second (n = 62) or third-generation (n = 64) oral contraceptives was observed using Dalbäck's assay (Curvers et al., 1999). More recently, a cycle-controlled cross-over study performed on 28 women who were not using oral contraceptives showed initially a more pronounced resistance to activated protein C induced by a 150 µg desogestrel-containing oral contraceptives than by a 150 µg levonorgestrel-containing oral contraceptive, both of them containing 30 µg ethinyl oestradiol (Tans et al., 2000
).
Thus, using Dalbäck's assay, an acquired low response to activated protein C is more likely to be associated with venous thrombosis in women on oral contraceptives. In normal asymptomatic women, a desogestrel-containing third-generation oral contraceptive induces a more important impairment of the plasma response to activated protein C than does the corresponding levogestrel-containing oral contraceptive. However, there is no evidence to show that this induced modification of plasma response to activated protein C is related to the clinical thromboembolic risk.
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Concerning the results of Rosing's assay in women taking second or third-generation oral contraceptives, in the absence of factor V Leiden mutation |
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The significance of higher basal ETP values in women on third-generation or triphasic oral contraceptives must be discussed. A basal ETP performed according to Rosing's technique has never been, in case-control or prospective studies, associated with an increased thrombotic risk. Among pregnant women who either do or do not carry the heterozygous factor V Leiden mutation, the basal ETP performed at 12, 22 and 34 weeks of amenorrhoea, or 3 months after delivery, do not significantly vary and show similar values in carriers and non-carriers of the mutation (Eichinger et al., 1999). In such circumstances, markers of haemostasis activation increase. The factor V Leiden gene mutation, however, is known as a real risk factor of thrombosis during pregnancy (Grandone et al., 1998
). Thus, the clinical relevance of Rosing's basal ETP deserves to be demonstrated.
A more fundamental criticism applies to the aPC-sr: there is no available clinical study showing that this parameter can indicate a thromboembolic risk. A case-control study nested in a vast German cohort studied the thromboembolic risk associated with abnormal aPC-sr values, using Rosing's test (Heinemann et al., 1998). In the absence of the factor V gene Leiden mutation, the aPC-sr does not indicate a clinical risk. Thus, acquired abnormal values of the aPC-sr do not have any clinical value.
Finally, using Rosing's test, women on third-generation oral contraceptives do not have any particularly abnormal acquired failure of the plasma anticoagulant activity induced by exogenous activated protein C. Third-generation oral contraceptives induce complex modifications of Rosing's test which do not depend on the absolute effect of activated protein C. Until now, these modifications have not been successfully related to the venous thromboembolic risk.
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Other questions concerning the response to activated protein C in women taking second or third-generation oral contraceptives, in the absence of factor V Leiden mutation |
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The aPC-sr parameter has been used to demonstrate the importance of the dose of levonorgestrel, a progestogen which is contained in some monophasic second-generation and triphasic oral contraceptives (Kluft et al., 1999). This work shows a negative effect of the mean daily dose of levonorgestrel on the aPC-sr parameter, the highest doses counteracting the increase of the aPC-sr induced by oral contraceptive intake. Thus, levonorgestrel induces plasma modifications able to modulate, in vitro, the results of a clinically non-informative biological test.
A recent cycle-controlled randomized cross-over trial aimed to compare the effect of a second-generation (progestogen: 150 µg levonorgestrel) with that of a third-generation (progestogen: 150 µg desogestrel) oral contraceptive, each of them containing 30 µg ethinyl oestradiol (Rosing et al., 1999). This elaborate study shows that the aPC-sr parameter increases with both compounds, with a higher increase in women on third-generation contraceptives. Increases of factor II, factor VII and thrombin-activated fibrinolysis inhibitor (TAFI), together with decreases of factor V and protein S were also shown. Very good positive linear correlations between basal and on treatment aPC-sr values have been demonstrated for both progestogens, with a higher slope for desogestrel (on treatment mean aPC-sr value of 4.5 for a basal value of 3.0) than for levonorgestrel (3.5 for 3.0). This indicates that the basal aPC-sr value depends on plasma parameters which are modified, with a mean intensity which depends on the type of progestogen, by the type of oral contraceptive. Thus, the basal aPC-sr value incorporates the value of the aPC-sr on oral contraceptives. The main criticism is that there is no available evidence that oral contraceptive-induced modifications of any biological parameter can be clinical risk indicators when their basal values (if any), have a documented status as an epidemiological indicator. A typical example is protein S, a co-factor of the protein C anticoagulant system. Its plasma concentrations decrease on oral contraceptives: studies have failed to demonstrate that it indicates a risk of thrombosis. We suspect that the variations of aPC-sr resulting from oral contraceptives merely reflect the numerous variations of the haemostasis factors induced by oestrogens and progestogens. Correlations between values of aPC-sr obtained before and while on oral contraceptives probably only reflect the complex positive and negative variations of coagulation factors induced by the various hormonal compounds. This has been confirmed in a recent cross-over study published by Rosing et al. (1999) demonstrating that the ETP-based assay is influenced by protein S plasma concentrations, particularly of free protein S, which are decreased by desogestrel but not by levonorgestrel (Tans et al., 2000
). Thus, so far, the impairment of the ETP-based aPC-sr observed with third-generation oral contraceptives and modulated by changes in protein S, have not yet been related to the thromboembolic risk. This discrepancy has recently been elegantly summarized (Kluft, 2000
): a drug may influence a global or non-specific test, sensitive to factors A to D, via a factor B that can show variations without clinical consequences. However, the variations in the test may also be associated with a clinical endpoint via its sensitivity to variations in factor D, when variations in factor D concentrations are clinically relevant. Thus, due to the factor which is influenced by a given drug, the same absolute variations of a non-specific test may be clinically relevant or not. Our goal is now to find the clinically-relevant factor D!
Finally, the aPC-sr parameter cannot yet be validated as an indicator of risk in asymptomatic women. A recent communication (Tans et al., 1999) of a work performed using the plasmas of the Leiden Thrombophilia Study has shown the existence of a higher venous thrombotic risk predicted by the aPC-sr parameter, but only in symptomatic men with a primary thrombotic accident. Moreover, this risk only appears in values of the aPC-sr parameter of the higher quartile compared to values of the lower quartile, the lower limit of the 95% confidence interval of the odds ratio being 1.1, i.e. close to 1. We have no data on asymptomatic women on oral contraceptives.
Oestrogens and androgens often have opposite hepatic effects, leading to opposite mean modifications of the plasma concentrations of various proteins: for instance, the sexual steroids binding protein, antithrombin, triglycerides, high density lipoprotein (HDL) cholesterol, etc. There is no evidence available to show that modifications of plasma concentrations induced by oestrogens have the same epidemiological significance as do basal concentrations. For instance, we currently do not know if the increase in HDL cholesterol induced by an oral oestrogen has the same protector effect against coronary diseases as does the basal HDL cholesterol concentration. Neither is there any available demonstration to show that the addition of the opposite effects, on plasma concentrations of an indicator of clinical risk, of an oestrogen and a progestogen leads to cancellation of the clinical risk associated with the modifications of the given biological indicator of risk induced by the oestrogen or progestogen alone. For instance, taking both an androgen and an oestrogen together leads to a decrease in the plasma HDL cholesterol concentrations, but we currently cannot affirm that it modifies the effect of oral oestrogens alone on atherogenesis, if any. It seems obvious, but not demonstrated, that oestrogens alone modify the basal value of the aPC-sr parameter, with no demonstration of a link between this modification and the induced thromboembolic risk. Taking an androgenic progestogen with oestrogens, probably lowers the induced modification of the aPC-sr (as for the other hepatic effects of oestrogens), without any demonstration that this ultimate modification, the `modification of q modification', lowers the risk of a hypothetical clinical consequence. The recorded dose effect of levonorgestrel backs up this interpretation. Third-generation progestogens, which have fewer androgenic properties, probably have a lower attenuation effect than second-generation progestogens on the basal aPC-sr value. Thus, it would seem very hazardous to claim that it induces, de facto, a more important thromboembolic risk than when an androgenic progestogen is used.
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Conclusions |
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Acknowledgments |
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Notes |
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This debate was previously published on Webtrack, www.oup.co.uk/humrep/commentSeptember 27, 2000
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References |
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