After adjusting for adipose tissue volume, which is known to be associated with fibrinogen levels, we found that type of ARV therapy was specifically associated with fibrinogen levels. Use of PIs as a group was associated with elevated fibrinogen levels. Subjects on PIs had 11% higher fibrinogen levels compared with those not on PIs. Koppel et al reported a similar increase when PI treated patients were compared to therapy naïve patients[8
]. The PI-associated increase in fibrinogen levels was independent of adipose tissue volumes, triglyceride levels, insulin resistance, inflammation, and lifestyle factors.
Unlike previous findings of PI-induced metabolic disturbances which have been observed with some, but not all, PI drugs [20
] elevation of fibrinogen levels here is seen with all PIs studied, suggesting a class effect. While we were not able to assess the association of more recently introduced PI drugs, we did find an association of the commonly used lopinavir/ritonavir combination and of ritonavir at boosting doses with higher fibrinogen levels. Most current PI regimens utilize ritonavir boosting.
The association between PIs and elevated fibrinogen levels may contribute to the risk of atherosclerosis. The Framingham study found that each SD increase in fibrinogen level (56 mg/dL) is associated with a 20% independent increment of risk for cardiovascular disease[27
]. Although it may not be possible to directly apply the Framingham cardiovascular risk assessment to our study population, the Framingham data suggest that in our study, subjects on PIs potentially have a 14% increased risk (39 mg/dl) for cardiovascular disease compared to subjects not on PIs. A similar magnitude of disease risk for PI was reported in the DAD study[3
] and by Kwong et al[4
]. Thus, elevation in fibrinogen levels may be a unifying mechanism by which PIs as a class accelerate atherosclerosis.
Fibrinogen levels were also independently associated with CRP, another cardiovascular risk factor which is also associated with acute inflammation. The lack of an association of PIs and NNRTIs with CRP levels [28
] and the finding that adding CRP to the multivariable analysis has virtually no effect on the association of PI with fibrinogen levels suggests that acute inflammation is not a central mechanism by which PI drugs are associated with elevated fibrinogen levels and atherosclerosis. The independent associations of PIs and CRP with fibrinogen levels, suggests that PIs may directly alter fibrinogen levels. It is possible that PIs and NNRTIs are instead more closely associated with regulation of the coagulation pathway itself. Traditional cardiovascular risk factor predictions do not take into consideration these potential effects of antiviral therapy on atherosclerosis.
In multivariable regression analysis, indinavir and ritonavir were the PIs most strongly associated with fibrinogen levels. Indinavir and ritonavir have also been shown to induce insulin resistance, and ritonavir induces hypertriglyceridemia.[21
] The combination of elevated fibrinogen levels and insulin resistance or hypertriglyceridemia may confer an even higher risk of atherosclerosis than individual disorder of metabolism alone. However, PIs that do not induce these metabolic changes were also associated with higher fibrinogen levels. Prospective studies comparing individual PI therapies and cardiovascular morbidity and mortality are required to assess the risk of cardiovascular disease with indinavir and ritonavir therapy compared to other PI therapy.
Ritonavir-boosting of indinavir was associated with higher fibrinogen levels than indinavir or low-dose ritonavir. The subjects on boosted indinavir regimens had the highest levels of fibrinogen in our study. Possible mechanisms include an additive effect of ritonavir itself on fibrinogen levels and/or a consequence of increasing the concentration of indinavir. The observation that high dose ritonavir was associated with higher levels of fibrinogen than lower dose ritonavir, albeit not statistically significant (p=0.30) supports the concept that total levels of PIs may be important in these effects. Further studies are necessary to assess the effects of individual PI dosage and ritonavir boosting on fibrinogen levels, as most PI regimens in common use at this time employ ritonavir boosting.
In contrast to PIs, NNRTIs were associated with lower fibrinogen levels. Subjects taking nevirapine and to a lesser extent efavirenz had lower fibrinogen levels compared to subjects not on NNRTIs. Interestingly, levels of fibrinogen with NNRTI therapy were also lower than those levels observed in healthy normal controls. The magnitude of the NNRTI effects changed very little after adjustment for lipids, including HDL levels. Koppel et al studied HIV-infected patients before the common use of NNRTI[8
]. While they found no decrease in fibrinogen levels in those who switched from PI to NNRTI, the numbers switching were very small (n= 23). It is possible that by lowering fibrinogen levels, NNRTIs confer a protective effect on cardiovascular events in addition to that predicted from their increase in HDL, but further prospective studies of NNRTIs and measures of cardiovascular outcomes are required.
As PIs have been commonly taken in combination with NNRTIs in the past, we examined the relationship between PIs, NNRTIs, and fibrinogen levels. Subjects on both NNRTIs and PIs had fibrinogen levels that were intermediate between those levels in subjects on PIs alone and NNRTIs alone and comparable to fibrinogen levels in healthy controls. The combined effect of PI and NNRTI therapy on fibrinogen levels strongly suggests a direct drug effect of both therapies.
The multivariable analysis found associations of higher HIV viral load and lower CD4 cell count with higher fibrinogen levels (data not shown). Unlike PIs and NNRTIs, HIV infection is also associated with elevated CRP levels, and thus elevated CRP and fibrinogen levels may be a reflection of the underlying inflammatory state due to HIV disease. It should be noted that the effects of PI and NNRTI persist after adjusting for these factors associated with HIV itself.
Several limitations of this study should be noted. Given the cross-sectional design of the present study, we are unable to prove causality between ARV therapy and alterations in fibrinogen levels. Prospective studies are required to determine the relationship between PIs, NNRTIs, and fibrinogen levels, and relationships to CVD events. Our study cannot assess the effects of more recently introduced PI drugs. The FRAM study is ongoing, and future analysis is planned of the association of newer PI and other ARV therapies with fibrinogen levels. Additional studies are clearly needed to evaluate the relationship between markers of CVD risk in HIV infection, ARV therapy, and cardiovascular morbidity and mortality.
In summary, PIs as a class are associated with higher levels of fibrinogen levels, which may contribute to an increased risk of atherosclerosis in HIV-infected subjects. Ritonavir boosting may lead to higher levels of fibrinogen compared to un-boosted PIs. In contrast, NNRTIs are associated with lower fibrinogen levels. Fibrinogen levels in those subjects on NNRTIs combined with PIs are similar to those observed in healthy controls. The antiretroviral drug effects appear to be independent of inflammation. Prospective studies are needed to determine the causal relationship between ARV therapy and fibrinogen levels as well as their CVD consequences.