2.3.2. Pharmaceutical Treatments for Cancer and Risk of Thrombosis
Chemotherapeutic agents have been used alone or in combinations for cancer therapy for about 80 years, with nitrogen mustard as the first agent. This was followed by the development of antifolates, such as methotrexate, which does not appear to have any toxic effect on endothelium [
52]. In the 1950s development of antimetabolites started with 6-mercaptopurine and 5-fluorouracil (5-FU). The latter has been associated with cardiotoxicity and coronary thrombosis [
53] or left ventricle thrombosis [
54]. Cwikiel et al showed that when 5-FU was incubated with endothelial cells the DNA synthesis decreased significantly [
52]. The same group demonstrated that 5-FU could disrupt the endothelial cell barrier, thereby exposing subendothelial matrix, with ensuing platelet aggregation [
52,
55]. It has, however, not been convincingly shown that treatment with 5-FU is associated with an increased risk of VTE.
The 1960s saw the development of anthracyclines, starting with daunorubicin, isolated from a Streptomyces strain. Subsequently, doxorubicin from a mutated variant, and analogues were developed. The anti-cancer mechanism of anthracyclines is by intercalation between adjacent DNA base pairs and inhibits topoisomerase II and thereby also the DNA-repair capacity. The anthracyclines are thrombogenic by several effects, that includes activation of coagulation, as evidenced by an increase in D-dimers and thrombin-antithrombin complexes, and this increase could be prevented with a prophylactic dose of LMWH [
56]. A research group at McMaster University, Canada, led by Patricia Liaw showed that in endothelial cells that were exposed to doxorubicin there was down-regulation of the mRNA of endothelial protein C-receptor (EPCR) and EPCR shedding, resulting in decreased EPCR levels on the endothelial surface and reduced activation of protein C [
57]. They thereafter demonstrated that anthracyclines increased the exposure of phosphatidylserine and expression of TF on endothelial cells as well as on monocytes [
58], which was also shown by Nigel Mackman’s group [
59]. Subsequently, they found that anthracyclines, as well as 5-FU, given to healthy mice increased the release of cell-free DNA, which correlated with an increase in thrombin-antithrombin complexes and in thrombin generation [
60]. Others have reported that patients treated with anthracyclines have decreased vasomotor reactivity, implying that the endothelial function was impaired compared to controls [
61]. All these prothrombotic alterations have clinical importance, since in a study of patients with multiple myeloma receiving two different chemotherapy regimens, one of which included doxorubicin, the latter was associated with a 6.5-fold increase in VTE [
62]. L-asparaginase was shown in 1963 to have antileukemic effect and was approved for medical use in the U.S. in 1978. It is used as first line treatment for acute lymphoblastic leukemia. Side-effects involving reduced level of antithrombin and development of VTE was reported already in 1979 [
63]. A meta-analysis of 17 studies including 1752 patients treated with L-asparaginase reported a VTE incidence of 5.2% (95% CI: 4.2-6.4) [
64]. Jacqueline Conard and others studied the mechanism for this and demonstrated that there is a decrease of antithrombin, protein C, protein S, FIX, FX, prothrombin and fibrinogen in plasma, with the natural anticoagulant reduction dominating initially, generating this prothrombotic state [
65,
66,
67].
The anticancer properties of cisplatin were accidentally discovered and the agent was introduced clinically in the 1970s. The mechanism of action of cisplatin and its analogs carboplatin, oxaliplatin and nedaplatin is through crosslinking of DNA, resulting in apoptosis of the tumor cells. Evidence of a procoagulant activity of cisplatin was demonstrated in 1990, based on TF-like activity from exposed monocytoid cells [
68]. Later, Lechner et al found that cisplatin caused release from endothelium of highly procoagulant microparticles, but the ensuing thrombin generation was driven by phospholipids rather than TF [
69]. Other reported procoagulant effects of cisplatin are upregulation of FXa and increased formation of thrombin on platelets [
70]. A meta-analysis of 38 trials with 8,216 patients showed that chemotherapy that included cisplatin increased the risk of VTE more than two-fold to 1.92% compared to 0.79% in controls, and the risk seems to be dose-dependent [
71].
Hormonal agents for cancer therapy were developed based on early observations of regression of breast cancer and prostate cancer after removal of the ovaries or testicles, respectively. Thus, in 1941 Huggins and Hodges initiated treatment of patients with prostate cancer with orchiectomy or estrogen [
72]. The goal is to achieve anti-androgen effect, with luteinizing hormone-releasing hormone (LHRH) synthetic agonists, gonadotropin-releasing hormone antagonsists (GnRH) and, in order to block the adrenal androgen production, the use of steroidal or non-steroidal antiandrogens, typically in combination [
73]. Hormonal therapies are also very effective for receptor-positive breast cancer, starting with tamoxifen, first produced in 1962, and with the mechanism of action through its anti-estrogenic effect in breasts, although it has estrogenic effects in the uterus and liver. It is thus a selective estrogen receptor modulator (SERM). The effects of tamoxifen on hemostasis are, similar to those of estrogen, prothrombotic with reduction of the natural anticoagulant levels (antithrombin, protein C, total protein S) and increase of coagulation factor activity (factors VIII, IX, VWF) [
74], and increased activated protein C resistance [
75]. Tamoxifen also contributes to platelet activation via activation of phospholipase Cγ and phosphoinositide-3-kinase, which results in release of intracellular free Ca2+ from the endoplasmatic reticulum [
76]. Aromatase inhibitors prevent the enzyme from converting androgen into estrogen in adipose tissue and are also used for the management of breast cancer. Anastrozole and letrozole belong to the family of selective aromatase inhibitors. A meta-analysis of 25 studies found that compared to the VTE prevalence of 0.5% in the general population, aromatase inhibitors had a VTE prevalence of 2.95%, although that was lower than for tamoxifen, OR 0.61 (95% CI, 0.37-1.00), driven by lower risk for DVT (OR 0.68) but not for pulmonary embolism (OR 1.01) [
77]. Although there is no evidence of aromatase inhibitor-associated changes in levels of coagulation factors and inhibitors [
78], an in vitro study showed that anastrozole decreased P-selectin expression and induced platelet aggregation and fibrin network formation [
79].
Immunomodulators include thalidomide, lenalidomide and pomalidomide. Although none of these agents seems to have any effect on coagulation on their own, when given together with dexamethasone for treatment of multiple myeloma, the CD62P increased on platelets and the closure time in the Platelet Function Analyzer-100 shortened, as well as increased levels of F VIII and soluble thrombomodulin [
80]. In a meta-analysis of 3,322 patients with multiple myeloma, treatment with thalidomide, dexamethasone and the combination increased the risk of VTE 2.6, 2.8 and 8-fold, respectively, whereas concomitant prophylactic dose anticoagulation eliminated that increase in risk [
81]. The VTE risk is similar with the second generation immunomodulators.
Growth factors that are targeted with monoclonal antibodies for cancer therapy include epidermal growth factor and its receptor (EGFR) and vascular endothelial growth factor (VEGF,
anti-angiogenic agents). A meta-analysis of 17 studies on the anti-EGFR agents cetuximab and panitumumab showed a 50% increase in risk of VTE compared to those receiving other regimens [
82], but the mechanism for hypercoagulability has not been elucidated. Although the VEGF-directed antibody bevacizumab was shown to increase expression of PAI-1 and the thrombus development in femoral vein injury or inferior vena cava obstruction in a mouse model [
83], the use of bevacizumab in 10 randomized clinical trials or of aflibercept in a meta-analysis did not reveal increased risk of VTE [
84,
85].
Kinase inhibitors include receptor tyrosine kinase inhibitors (RTKI) and cyclin-dependent kinase inhibitors (CDK). Whereas the first generation TKI, imatinib, was not associated with increased risk of VTE, the second generation agents, the breakpoint cluster region protein-tyrosine kinase protein ABL1 derivatives, such as ponatinib increased expression of VWF and platelet adhesion with microvascular angiopathy [
86]. In a meta-analysis of randomized controlled trials the second generation TKIs (ponatinib, nilotinib, dasatinib) were mainly associated with arterial thromboembolism, but there was also a 3-fold increase in venous occlusive events compared to imatinib [
87]. Of the CDK inhibitors it seems like mainly abemaciclib is associated with an increased risk of VTE but the pathophysiology behind this effect has not been revealed [
88].