Multiple Myeloma Treatment Is Associated with Enhanced Platelet Reactivity

Multiple myeloma (MM) is associated with elevated levels of thrombotic disease. Poor mobility due to bone disease and older age are contributory factors but a satisfactory molecular or cellular explanation for this has not been established. Furthermore, the treatment of newly-diagnosed or relapsed MM patients with immunomodulatory drugs increases thrombosis risk. In this study, we therefore sought to determine whether platelet function and/or haemostasis is altered during the clinical progression from the benign monoclonal gammopathy of underdetermined significance (MGUS) to the smouldering myeloma (SM, untreated) state, and to establish the impact of lenalidomide therapy in MM patients on platelet reactivity.

Patients (MGUS (N=10), SM (N=10) MM (N=4), healthy volunteer/control (N=10)) were recruited at the myeloma clinics at the Royal Berkshire and Oxford University Hospital Trusts. Single blood samples were obtained from controls, MGUS and smoldering myeloma participants. For patients undergoing therapy (lenalidomide, dexamethasone and anti-coagulant or anti-platelet therapy), samples were taken at predose Cy1, Cy1 D14, Cy1 D28 (a week free from lenalidomide administration), and Cy4 D14. All blood samples were taken less than 12 hours from Lenalidomide dosing, and analysis performed within 4 hours. High throughput platelet function analysis was performed on platelet-rich plasma and washed platelets to measure platelet aggregation, secretion and fibrinogen binding in response collagen, collagen-related peptide (a collagen receptor GPVI-selective ligand), ADP, thrombin receptor activatory peptide (TRAP), the thromboxane mimetic U46619 and epinephrine, and full concentration-response profiles obtained for each. The levels of key platelet receptors including GPVI, CD61, CD41b, CD36 and CD31, were also measured by flow cytometry and remained unaltered between groups. In comparison with control samples, platelet functional responses (maximal response, maximal gain and EC₅₀) from MGUS and smoldering myeloma patients were unaltered (EC₅₀: ADP 1µM, collagen-related peptide (CRP-XL) 0.1µg/ml, U46619 1µM, TRAP 1.6µM, epinephrine 1.3µM). Similarly circulating levels of platelet-bound fibrinogen and P-selectin exposure were unaltered (control: 709±194 AU, MGUS: 581±129 AU, SM: 655±148 AU 9 (Mean±SD)). In MM patients during therapy, the levels of these platelet responses (aggregation, fibrinogen binding and P-selectin exposure - a marker for a-granule secretion) were increased 2-4 fold. In most patients this was notable at week 2 and had declined approximately 50% by week 4 of cycle 1 but remained above predose levels, and remained elevated to a similar level in drug cycle 4.

The study of further patients (including Bortezomib treated patients as controls) will be required in order to validate these findings which suggest that (1) baseline platelet function is not substantially compromised in MM and SM, (2) lenalidomide and other immunomodulatory drugs may be able to modulate platelets in patients causing increased platelet reactivity, and (3) effects noted are lenalidomide-dependent and not related to inflammatory state secondary to disease response. The effects of exposure of platelets to lenalidomide in vitro in the presence or absence of dexamethasone and a proteasome inhibitor are being tested to explore potential direct effects of the drugs on circulating platelets. Results from these experiments will also be presented.