Variable serotonin release assay pattern and specificity of PF4-specific antibodies in HIT, and clinical relevance

BACKGROUND

The diagnosis of heparin-induced thrombocytopenia (HIT) requires functional assays to demonstrate that platelet factor 4 (PF4)-specific antibodies activate platelets, typically when therapeutic heparin (H) concentrations are tested ("classical" pattern). Some HIT samples also activate platelets without heparin ("atypical" pattern), but with unclear clinical significance.

OBJECTIVES

We aimed to assess whether platelet activation pattern and some characteristics of PF4-specific antibodies were associated with the severity of HIT.

PATIENTS/METHODS

Serotonin release assay (SRA) pattern of 81 HIT patients were analyzed and compared with their clinical and biological data, including levels of anti-PF4/H immunoglobulin G (IgG) and anti-PF4 IgG in 47 of them.

RESULTS

Higher anti-PF4/H IgG titers were measured in patients with an "atypical" SRA (optical density 2.52 vs. 1.94 in those with a "classical" pattern, p < .001). Patients of both groups had similar platelet count (PC) nadir and time to recovery, but those with an "atypical" SRA more frequently developed thrombotic events (69% vs. 34%, p = .037). Significant levels of anti-PF4 IgG were detected in both groups (38% and 61%, respectively). Whatever the SRA pattern, a lower PC nadir (35 vs. 53 G/L, p = .006) and a longer PC recovery time (6 vs. 3 days, p = .015) were evidenced in patients with anti-PF4 antibodies, compared with those with anti-PF4/H IgG only.

CONCLUSIONS

An atypical SRA pattern with elevated anti-PF4/H IgG titers seems associated with an increased risk of thrombosis in HIT. IgG antibodies to native PF4 may contribute to more severe and persistent thrombocytopenia, and their detection could be useful in clinical practice.

The deglycosylated form of 1E12 inhibits platelet activation and prothrombotic effects induced by VITT antibodies

In order to improve the safety of COVID-19 vaccines, there is an urgent need to unravel the pathogenesis of vaccine-induced immune thrombotic thrombocytopenia (VITT), a severe complication of recombinant adenoviral vector vaccines used to prevent COVID-19, and likely due to anti-platelet factor 4 (PF4) IgG antibodies. In this study, we demonstrated that 1E12, a chimeric anti-PF4 antibody with a human Fc fragment, fully mimics the effects of human VITT antibodies, as it activates platelets to a similar level in the presence of platelet factor 4 (PF4). Incubated with neutrophils, platelets and PF4, 1E12 also strongly induces NETosis, and in a microfluidic model of whole blood thrombosis, it triggers the formation of large platelet/leukocyte thrombi containing fibrin(ogen). In addition, a deglycosylated form of 1E12 (DG-1E12), which still binds PF4 but no longer interacts with Fcγ receptors, inhibits platelet, granulocyte and clotting activation induced by human anti-PF4 VITT antibodies. This strongly supports that 1E12 and VITT antibodies recognize overlapping epitopes on PF4. In conclusion, 1E12 is a potentially important tool to study the pathophysiology of VITT, and for establishing mouse models. On the other hand, DG-1E12 may help the development of a new drug that specifically neutralizes the pathogenic effect of autoimmune anti-PF4 antibodies, such as those associated with VITT.

Multicenter studies on the pharmacokinetic profile of sustained-release oral diltiazem (300 mg) after once a day repeated administration: influence of age

The influence of age on the pharmacokinetics of the oral sustained release diltiazem Mono-Tildiem LP 300 mg was investigated in 12 middle-aged (40-64 years), 12 elderly (65-80 years) patients and compared to a control group of 54 young healthy volunteers (18-36 years). Each subject received daily a single dose of diltiazem slow release (300 mg) in the morning for 5 consecutive days. On the fifth day of treatment, the pharmacokinetic parameters of diltiazem and of 2 of its circulating metabolites (N-monodemethyldiltiazem and deacetyldiltiazem) were evaluated. The mean diltiazem Cmax was 199.3 +/- 117.8 ng/ml, 254.8 +/- 85.2 ng/ml and 154.5 +/- 63.2 ng/ml in middle-aged, elderly, and young healthy subjects, respectively. Mean plasma Cmin concentration was also higher in elderly subjects than in middle-aged and young subjects: 129.7 +/- 77.9 ng/ml versus 66.8 +/- 56.8 and 66.3 +/- 32.3 ng/ml, respectively. The AUC0-24 showed the same trend: 4,042 +/- 1,136 ng/ml.h in elderly, 2,995 +/- 1,905 ng/ml.h in middle-aged, and 2,564 +/- 1,205 ng/ml.h in young subjects. These parameters were statistically higher (p < 0.01) in the elderly subjects than those obtained in younger people. No statistical difference was observed between young volunteers and middle-aged patients. The Tmax did not differ significantly with age (5.1 +/- 4.4, 6.8 +/- 2.8, 6.1 +/- 3.5 hours, respectively). The ratios between the AUC of each metabolite and that of the parent compound did not vary with the age. These results suggest that in elderly people (> 65 y) the bioavailability of diltiazem is increased, probably due to a reduction of the first-pass effect. Based on the pharmacokinetic results, although safety data did not show any specific trend with age, a precautionary reduction of the dose at the start of the treatment seems advisable.