Advanced diagnostics and novel therapeutics for platelet function disorders

Platelet function plays a critical role in hemostasis, and dysfunction in this mechanism
is implicated in a range of bleeding disorders. The most rare inherited platelet function
disorders (IPFDs) are relatively easy to diagnose. However, the more prevalent IPFDs
often show a high diagnostic difficulty, leading to underdiagnosis. In terms of
treatment of IPFD patients, no prophylactic treatment is available, causing high
patient burden and relatively low quality of life for patients suffering from a severe
IPFD. This thesis describes novel diagnostic and therapeutic approaches for this
group of disorders.

Advanced diagnostics …
In Chapter 2, we described the characterization of a tetrameric nanobody-based
glycoprotein VI (GPVI) platelet agonist and compared it to current agonists used in
platelet diagnostics, that are hard to standardize and difficult to produce. We showed
that our nanobody is able to discriminate between healthy controls and GPVI-related
platelet disorders to a similar extent as collagen or cross-linked collagen-related
peptide (CRP-XL). In this way, we provide a more stable, easier to produce alternative
to current GPVI agonists, which is beneficial for standardization of platelet diagnostics.

In Chapter 3, we have validated a novel whole blood-based rapid test for delta
storage pool disease (δ-SPD), based on platelet ATP secretion. The current standard
for diagnosis of δ-SPD is laborious, has a long turn-around time and requires large
volumes of blood. We aimed to introduce a microarray that decreases the labor
intensiveness, turn-around time and required volume of blood. With its high sensitivity,
this test is able to exclude δ-SPD, decreasing the need to send all patients for more
advanced diagnostic techniques.

In Chapter 4, we have compared the platelet proteome of healthy controls and
people with Glanzmann thrombasthenia (GT). We have shown that platelets from
GT patients show downregulation of several platelet specific α-granule proteins.

… and novel therapeutics for inherited platelet function disorders
In Chapter 5, we introduced a potential novel prophylactic treatment for inherited
bleeding disorders. HMB-001 is a bispecific antibody that recognizes (activated)
coagulation factor VII (FVII(a)) and TREM-like transcript-1 (TLT-1) on the activated
platelet surface, thereby targeting FVIIa to the platelet surface and boosting fibrin
formation to compensate for defects in primary hemostasis. Our preclinical data has
shown that HMB-001 is effective in enhancing fibrin formation ex vivo, prolonging
endogenous FVIIa half-life in nonhuman primates and potentiating FVIIa hemostatic
activity in mouse bleeding studies. HMB-001 has therefore potential to offer subcutaneous
prophylactic treatment to prevent bleeds in people with GT and other inherited
bleeding disorders, with a low-frequency dosing regimen.