Therapy for PV aims to reduce the risk of thrombosis and bleeding, to control symptoms, to delay transformation to myelofibrosis (MF) or acute leukemia/myelodysplastic syndromes (MDS), and to manage special situations [
3,
41]. Given the high mortality associated with thrombotic events in patients with PV, the first goal of therapy is to reduce the risk of thrombosis, mainly by controlling HCT to < 45% [
15], a target associated with reduced rates of cardiovascular death and major thrombosis [
13]. Therapy for the treatment of PV is dependent on the patient’s thrombotic risk, which is currently based on age and history of thrombosis [
15,
30,
42]. Patients < 60 years old with no history of thrombosis are categorized as low risk, whereas those ≥ 60 years old and/or those with a history of thrombosis are considered high risk [
15]. Current guidelines recommend managing low-risk patients with phlebotomy and low-dose aspirin, whereas high-risk patients should be treated with cytoreductive agents, with hydroxyurea and recombinant interferon alfa as first-line therapies and interferon and ruxolitinib as second-line therapies in patients who are intolerant of or have inadequate response to hydroxyurea [
15].
However, findings from a recent retrospective study by Barbui and colleagues suggest that there may be a role for cytoreductive therapy in the primary prevention of TEs in some patients with low-risk PV [
18]. In this study, 604 patients with low-risk PV were treated with aspirin and phlebotomy (median duration, 4.9 years) to keep the target HCT < 45%; however, 12% of patients experienced 84 major thrombotic events (venous, 45%; arterial, 55%). Arterial hypertension was significantly associated with a higher rate of arterial events in these patients, suggesting that patients with low-risk PV with arterial hypertension may require more intensive therapy, including cytoreductive therapy and/or antihypertensive treatments, such as angiotensin-converting-enzyme inhibitors [
18]. However, prospective studies are needed to assess the most appropriate therapy.
In addition to cytoreduction, antiplatelet agents are generally used to treat patients with a history of arterial thrombosis, and those with a history of venous events are treated with anticoagulants (e.g., vitamin K antagonists [VKAs]) [
43]. Findings from a recent study showed the benefits associated with the use of cytoreductive therapy in combination with antithrombotic drugs in patients with a history of TEs. This study of 597 patients with MPNs (PV,
n = 184) examined the benefit-risk profile of cytoreductive drugs along with antiplatelet and antithrombotic therapies that were started after an initial TIA (
n = 270; PV,
n = 77) or ischemic stroke (
n = 327; PV,
n = 107) [
42]. Treatment included antithrombotic therapy (aspirin, 85% of patients) and cytoreductive drugs (hydroxyurea, 78% of patients). The composite incidence of recurrent TIA and ischemic stroke, acute myocardial infarction, and cardiovascular death was 4.2% and 19.2% at 1 and 5 years after the index event, respectively, which was lower than that in the general population. Cytoreductive therapy was a strong protective factor (HR, 0.24), and the rate of major bleeding was similar to that in the general population (0.90 per 100 patient-years), suggesting an advantageous benefit-risk profile of cytoreductive and antithrombotic therapy [
42].
Similarly, cytoreduction in combination with oral anticoagulants may also help prevent the recurrence of thrombosis, especially venous thrombosis, in patients with PV [
5,
43‐
45], with one study reporting a 2.8-fold reduction in the risk of thrombotic recurrence with VKA treatment [
43]. In a retrospective study that examined the rate of recurrence of arterial and venous thrombosis in 494 patients (PV,
n = 235; essential thrombocythemia [ET],
n = 259) with previous arterial (67.6%) or venous (31%) thrombosis, cytoreduction was the only treatment significantly associated with a reduction in the risk of recurrence (multivariable HR, 0.53 [95% CI, 0.38–0.73];
P = .0002) [
44]. However, patients treated with oral anticoagulants plus cytoreduction had the lowest rate of recurrences (17.8%) compared with those treated with cytoreduction (50.0%), antiplatelet agents (35.2%), or anticoagulation alone (44.1%). When stratified by type of first event (i.e., arterial vs venous), cytoreductive treatment was associated with a significant decrease in recurrence of arterial thrombosis (HR, 0.47 [95% CI, 0.31–0.70];
P = .0003), whereas anticoagulants (HR, 0.32 [95% CI, 0.15–0.64];
P = .001) or antiplatelet therapies (HR, 0.42 [95% CI, 0.22–0.77];
P = .006) were associated with a significant decrease in the risk of recurrent venous thrombosis [
44]. A study by De Stefano and colleagues (
n = 206; PV, 46.6%) reported similar findings, with a lower incidence rate of recurrent venous thrombosis per 100 patient-years observed in patients receiving VKAs (4.7 [95% CI, 2.8–7.3] vs 8.9 [95% CI, 5.7–13.2];
P = .03) [
45]. Duration of treatment was also assessed, with findings suggesting that long-term treatment may lead to lower incidence rates of recurrence per 100 patient-years compared with stopping VKA treatment (5.3 [95% CI, 3.2–8.4] vs 12.8 [95% CI, 7.3–20.7];
P = .008) [
45]. The benefits of prolonged treatment with anticoagulants in patients with MPNs were also observed in the study by Wille et al. [
5]. In this study, recurrent venous TEs were observed in 36.1% of patients who terminated prophylactic anticoagulation and in only 8.6% of patients who continued anticoagulation therapy (
P = .0127). Most patients with recurrent venous TEs (81.3%) were not receiving anticoagulants at the time of recurrence. Given that bleeding complications are a major concern among patients taking anticoagulation, physicians may recommend shortening the duration of treatment with anticoagulants. However, in these studies, treatment with anticoagulants did not significantly increase the incidence of major bleeding, supporting long-term use of anticoagulants such as VKAs in patients with MPNs who have a history of thrombotic events [
5,
43‐
45].
Aspirin and phlebotomy
Phlebotomy is one of the recommended first-line treatments for patients with PV [
13,
15]. Phlebotomy helps control HCT, with the goal of maintaining HCT to < 45% [
13]. However, a study evaluating the need for additional phlebotomies in 533 patients with PV who were receiving hydroxyurea treatment showed that a higher intensity of treatment with phlebotomy was related to an increased risk of thrombotic events: patients requiring ≥ 3 phlebotomies per year had a higher risk of thrombosis compared with patients needing ≤ 2 phlebotomies per year (20.5% vs 5.3% at 3 years;
P < .0001) [
46]. However, a recent analysis of the ECLAP and CYTO-PV studies suggested that there is no correlation between the intensity of the phlebotomy regimen and the risk of thrombosis in patients with PV [
47].
The ECLAP study demonstrated that treatment with aspirin prevented thrombotic complications in patients with PV [
4]. Low-dose aspirin reduced the risk of nonfatal myocardial infarction, nonfatal stroke, pulmonary embolism, major venous thrombosis, and death from cardiovascular causes (HR, 0.40 [95% CI, 0.18–0.91];
P = .03). Consistent with these findings, in the ECLAP study, antiplatelet therapy was significantly associated with a lower risk of cardiovascular events (HR, 0.72 [95% CI, 0.53–0.97];
P = .0315) [
3].
Hydroxyurea
Hydroxyurea is the most commonly used first-line cytoreductive therapy in patients with PV [
15]. This practice is based mainly on studies conducted by the Polycythemia Vera Study Group (PVSG) and the French Polycythemia Study Group [
41,
48,
49]. The PVSG study was conducted in 51 patients with PV who were all treated with hydroxyurea, and its efficacy was compared retrospectively with that in 194 patients treated with phlebotomy only [
50]. Hydroxyurea treatment led to a reduction in the number of thrombotic events (9.8% vs 32.8% in the phlebotomy group;
P = .009). The French Polycythemia Study Group compared hydroxyurea therapy with pipobroman therapy in a randomized study of 292 patients with PV who were < 65 years old, with a median follow-up of 9 years [
48]. Initially, each therapy led to a complete hematologic remission in all but 5 patients (pipobroman,
n = 3; hydroxyurea,
n = 2). In the long-term analyses of this study, no significant differences in the incidence of thrombosis were seen between the 2 therapies, but the risk of leukemic transformation was clearly higher in the pipobroman arm. The final results of this trial showed that, with a median follow-up of 16 years, pipobroman presented a very high risk of evolution to acute leukemia/MDS (cumulative incidence of 52% at 20 years vs 24% with hydroxyurea) and that evolution to acute leukemia/MDS was the most common cause of death in this cohort of patients [
41].
More recently, Barbui and colleagues examined 1042 patients included in the ECLAP study, during the follow-up phase (median, 2.8 years), who received phlebotomy only (
n = 342) or hydroxyurea only (
n = 681) to maintain an HCT of < 45% [
51]. A lower incidence of fatal and nonfatal cardiovascular events was reported in the hydroxyurea group than in the phlebotomy group (3.0 vs 5.8 per 100 patient-years, respectively;
P = .002) [
51]. In addition, in the high-risk group (> 60 years and/or prior history of thrombosis), treatment with hydroxyurea was associated with a significantly lower rate of fatal and nonfatal cardiovascular events (4.8 vs 8.7 per 100 patient-years), hematologic transformations (0.1 vs 1.5 per 100 patient-years), and overall mortality (0.1 vs 0.5 per 100 patient-years) compared with phlebotomy alone [
51]. However, as mentioned previously, cytoreductive therapy alone may not be sufficient to prevent recurrent thrombosis. In the study by Wille and colleagues, only 25% of recurrences of venous TEs occurred when patients were not receiving cytoreductive treatment [
5]. Interestingly, hematologic parameters were controlled, suggesting that the addition of anticoagulation therapy to cytoreduction is important in preventing venous TEs. Importantly, no significant increase in major bleeding was observed in patients who received concomitant anticoagulation and cytoreduction.
Although hydroxyurea treatment lowers the rate of cardiovascular events, approximately 15–24% of patients may eventually become resistant to or experience unacceptable adverse effects from this treatment (hydroxyurea intolerance) [
35,
52]. Resistance is important to recognize since it is associated with higher risk of death and transformation. The ELN has published criteria for identifying patients experiencing clinical resistance to or intolerance of hydroxyurea [
53]. Cytopenias, uncontrolled myeloproliferation, and increased phlebotomy requirements are associated with hydroxyurea resistance, and skin toxicity, mucocutaneous toxicity, gastrointestinal toxicity, and fever are associated with hydroxyurea intolerance [
35].
Skin toxicity, one of the more common adverse events associated with hydroxyurea treatment, has been reported in approximately 5–11% of patients with MPNs [
54‐
56]. In a retrospective study evaluating severe mucocutaneous toxicity associated with hydroxyurea in 614 patients with MPNs (PV, 34.9%), 51 patients (8.3%) reported skin toxicity after a median treatment period of 32.1 months [
55]. In patients with PV, 35.3% reported experiencing skin toxicity; however, a similar proportion did not (34.8%;
P = .53). Permanent discontinuation of hydroxyurea was reported in 27 patients (52.9%) overall [
55]. In a large retrospective study of 3411 patients with MPN (PV,
n = 963), 536 patients were treated with hydroxyurea and evaluated for drug-related toxicities [
56]. Hydroxyurea-related toxicities were reported in 184 patients (5%; PV,
n = 61 [33%]), which included mucocutaneous lesions (
n = 167 [90.8%]; PV,
n = 57 [94%]) [
56]. The overall discontinuation rate due to hydroxyurea toxicity was 5%. This is lower than discontinuation rates previously reported, including rates observed in the UK Medical Research Primary Thrombocythemia 1 study in high-risk ET (10.6%) [
54]. However, gastrointestinal toxicities were not reported in the retrospective study but were reported in the Primary Thrombocythemia 1 study, which may have contributed to the difference in discontinuation rates. More recent prospective, albeit smaller, studies suggest that rates of hydroxyurea-related skin toxicity in patients with MPNs may be higher. A prospective, noninterventional study conducted in Germany found that 43% of patients with MPNs (PV,
n = 55; ET,
n = 55; MF,
n = 41) exposed to hydroxyurea (median exposure, 46 months) presented with skin abnormalities compared with 7% of patients treated with other therapies (ruxolitinib, anagrelide, or pegylated interferon alfa;
P = .0001) [
57]. Overall, 13% of patients discontinued due to skin toxicity vs 2% of patients who were not treated with hydroxyurea (
P = .014). Another prospective, single-center study assessed the incidence of cutaneous adverse events in patients with ET (
n = 74) or PV (
n = 36) treated with hydroxyurea and reported that, overall, 60% of patients (66 of 110) experienced a cutaneous adverse event, with 54% of those patients (36 of 66) developing a serious cutaneous adverse event [
58]. At 48 months, the cumulative incidence was 70% for any cutaneous adverse event and 20% for any serious cutaneous adverse event. Overall, adverse events and discontinuation rates due to hydroxyurea therapy were relatively low in retrospective studies but were more frequently reported when prospectively tracked; therefore, physicians need to be aware that skin toxicities with hydroxyurea may be more frequent than expected and can be severe [
54,
56] and that dermatologic monitoring is recommended in these patients, especially in those who present with actinic keratoses or a history of squamous cancer before the initiation of hydroxyurea.
Interferon
Interferon has been shown to induce high rates of hematologic and molecular responses in patients with PV [
59,
60] and is recommended as frontline therapy, especially for young patients who need long-term treatment, and as second-line therapy for patients with PV who are intolerant of or have inadequate response to hydroxyurea [
15,
61]. Interferon has been evaluated in several small studies, including some phase 2 studies, in which it has been shown to be effective in achieving hematologic remission, reducing
JAK2 V617F allele burden, and reducing rates of thrombosis [
62‐
64]. Discontinuation occurs in approximately 25% of patients, and tolerability is improved with the use of low doses at initiation. In some patients, interferon may achieve sustained hematologic and molecular responses even after discontinuation of therapy.
PROUD-PV (NCT01949805), a randomized, controlled, multicenter, phase 3 trial comparing the efficacy, safety, and tolerability of hydroxyurea and ropeginterferon alfa-2b in 257 patients with PV who were not resistant to or intolerant of hydroxyurea showed noninferiority of ropeginterferon alfa-2b compared with hydroxyurea in terms of complete hematologic response according to ELN criteria, with spleen normality at 12 months [
65,
66]. Forty-five percent of patients had a hematologic response, with mean HCT decreasing from 48 to 42%, leukocyte counts decreasing from 12 to 6 × 10
9/L, and platelet counts decreasing from 530 to 260 × 10
9/L. The need for phlebotomy within 3 months decreased from 86 to 6%. A
JAK2 molecular response was achieved in 37% of patients, with mean mutant
JAK2 allele burden decreasing from 42.5 to 28.7%. However, observed spleen reductions with ropeginterferon were not clinically relevant due to the almost-normal baseline spleen size in the majority of patients. Overall, ropeginterferon alfa-2b had a better adverse event profile compared with hydroxyurea and was well tolerated. Although more patients in the ropeginterferon alfa-2b group experienced cardiovascular events (3.1%, including cardiac failure, thrombotic event, and stroke), endocrine events (3.1%, including autoimmune thyroiditis and hypo- or hyperthyroidism), or psychiatric events (1.6%, including anxiety, depression, and mood altered), the latter being a well-known toxicity of interferon, the incidence of these events was not statistically significant compared with that in the hydroxyurea group. A 12-month continuation of this study (CONTINUATION-PV; NCT02218047) comparing ropeginterferon alfa-2b with best available therapy (BAT) showed that, after 24 months of treatment, complete hematologic response (CHR) rates were higher in the ropeginterferon alfa-2b group compared with the BAT group (CHR, 70.5% vs 49.3%, respectively;
P = .01); however, cardiovascular and vascular disorders occurred at a rate of 10.2% in the ropeginterferon alfa-2b group and 5.5% in the BAT group. Overall, treatment-related adverse events were reported in 70% and 77% of patients treated with ropeginterferon alfa-2b and BAT, respectively [
67].
Ruxolitinib
Ruxolitinib is the only JAK inhibitor approved for the treatment of patients with PV, specifically those who are resistant to or intolerant of hydroxyurea [
15,
68]. Ruxolitinib was evaluated in 2 phase 3 studies in patients who were resistant to or intolerant of hydroxyurea and had splenomegaly (RESPONSE; NCT01243944 [
69]) or no palpable spleen (RESPONSE-2; NCT02038036 [
70]). Both studies met their primary endpoints and showed that ruxolitinib was superior to BAT in providing HCT control without phlebotomies and improving symptom burden in this patient population, regardless of spleen size. In the 208-week (4-year) analysis of the RESPONSE study, 37% of patients were still receiving treatment with ruxolitinib vs no patients in the BAT arm [
71].
Although the RESPONSE and RESPONSE-2 studies were not powered to assess TEs, fewer thrombotic events were seen in patients treated with ruxolitinib compared with BAT. In the RESPONSE study, thrombotic events occurred in 1 patient (0.9%) treated with ruxolitinib and 6 patients (5.4%) treated with BAT (1.8 vs 8.2 per 100 patient-years of exposure, respectively) [
69,
72]. In a 4-year analysis of the RESPONSE study, the rate of TEs was lower with ruxolitinib compared with BAT (all grades, 1.2 vs 8.2 per 100 patient-years; grade 3/4, 0.7 vs 2.7 per 100 patient-years, respectively) [
71]. In the primary analysis of RESPONSE-2, the corresponding rates were 1.4% (
n = 1) with ruxolitinib and 4.0% (
n = 3) with BAT [
70]. At 80 weeks of follow-up in RESPONSE-2, embolic and thrombotic events occurred at a rate of 1.5 per 100 patient-years in the ruxolitinib group and 1.9 per 100 patient-years in the BAT group [
73]. This finding may be attributed to better HCT and WBC control with ruxolitinib than with standard therapy, given that these 2 hematologic parameters have been independently linked to an increased risk of thrombotic events [
13,
28]. In the primary analysis of the RESPONSE studies, the proportion of patients who achieved HCT control (i.e., ≤ 45%) was significantly higher with ruxolitinib than with BAT (RESPONSE, 60.0% vs 18.8%; RESPONSE-2, 62.0% vs 19.0%) [
69,
70]. HCT control was also maintained in most patients (RESPONSE, 73% for 208 weeks; RESPONSE-2, 78% for 80 weeks) [
71,
73]. Additionally, in both RESPONSE studies, the proportion of patients undergoing phlebotomy procedures was lower with ruxolitinib than with BAT. This finding could be important in assessing the risk of thrombosis given that, as described above, the intensity of treatment with phlebotomy may be related to an increased risk of thrombotic events [
46].
In the RESPONSE study, ruxolitinib also led to control of WBC counts in patients with PV. A subanalysis of the RESPONSE study showed that ruxolitinib led to greater reductions in WBC counts compared with BAT or hydroxyurea. In patients with baseline WBC counts of ≥ 11 × 10
9/L, those treated with ruxolitinib had greater mean reductions in WBC counts compared with those treated with BAT, and these reductions were maintained over time [
74]. Among patients with WBC counts of > 10 or > 15 × 10
9/L at baseline, a higher proportion of ruxolitinib-treated patients achieved an ELN response (WBC count ≤ 10 × 10
9/L) [
74]. In addition to these analyses, a meta-analysis of the COMFORT-I, COMFORT-II, and RESPONSE studies evaluated the effect of ruxolitinib on the risk of thrombosis among patients with MF or PV [
75]. The rates of thrombosis were significantly lower in patients who were treated with ruxolitinib (risk ratio, 0.45 [95% CI, 0.23–0.88]). The rates of venous and arterial thrombosis also demonstrated similar risk ratios (0.46 [95% CI, 0.14–1.48] and 0.42 [95% CI, 0.18–1.01], respectively); however, these risk ratios did not reach statistical significance.
However, ruxolitinib was associated with an increased rate of herpes zoster infection compared with standard therapy (RESPONSE: exposure-adjusted rate at 4 years, 4.9 per 100 patient-years; RESPONSE-2: exposure-adjusted rate at 80 weeks, 3.8 per 100 patient-years); most herpes zoster infections were grade 1 or 2 and resolved without sequelae [
71,
73]. Rates of nonmelanoma skin cancer were also increased in patients who received ruxolitinib (RESPONSE: exposure-adjusted rate at 4 years, 3.6 per 100 patient-years; RESPONSE-2: exposure-adjusted rate at 80 weeks, 0.8 per 100 patient-years for squamous cell carcinoma of skin only) [
71,
73]. Prior nonmelanoma skin cancer, previous therapy (e.g., hydroxyurea) or aging may have had an impact on the nonmelanoma skin cancer rates observed with ruxolitinib. This finding was described in the 80-week follow-up data from the RESPONSE study, in which nonmelanoma skin cancers were observed in the originally randomized ruxolitinib arm, primarily in patients with a history of nonmelanoma skin cancer. However, at the 80-week analysis, exposure-adjusted rates were generally similar between the ruxolitinib and BAT arms [
76]. Furthermore, all patients who developed squamous cell carcinoma of the skin in the RESPONSE-2 study at 80 weeks had prior exposure to antineoplastic therapy, including hydroxyurea [
73]. It has recently been reported that there may be an increased risk of developing B cell lymphomas in patients with MF treated with ruxolitinib, in particular those presenting with a clonal immunoglobulin gene rearrangement in the bone marrow before starting ruxolitinib [
77]; however, there have been no reports of B cell lymphomas in patients enrolled in the RESPONSE studies in PV. Additional studies are needed to determine the risk in this population.
Treatment options for splanchnic vein thrombosis
MPNs are a leading cause of noncirrhotic and nonmalignant splanchnic vein thrombosis (SVT) [
78]. SVT is a rare type of venous thrombosis that may involve several abdominal veins (portal, splenic, mesenteric, and hepatic) and includes Budd-Chiari syndrome, extrahepatic portal vein obstruction, and mesenteric vein thrombosis [
79]. SVT is seen in all types of MPNs and is mainly observed in younger patients [
78,
80,
81]. PV is the most common MPN subtype in patients with SVT [
82], occurring in 0.8–2% of patients with PV [
10,
12].
JAK2 V617F is common in patients with SVT and has been detected in 96.5% of patients with SVT and MPNs and in 7% of patients with SVT who have no MPN features on bone marrow biopsy [
81]. Overall, SVT has been reported to account for 7.5% of first thromboses in patients with MPNs [
44].
Management of SVT in patients with MPNs may be challenging and is usually focused on preventing recurrent thrombosis, managing MPNs, and managing organ dysfunction [
80]. If there are no major contraindications, anticoagulant therapy is usually recommended for all patients presenting with acute symptomatic splanchnic vein thrombosis [
80,
83]. Typically, patients are started on either a full-dose low-molecular-weight or unfractionated heparin followed by VKA [
80,
83]. However, the use of anticoagulant therapy should be carefully monitored given the increased risk of bleeding, which must be balanced against the need to prevent thrombosis recurrence. Patients with PV and SVT should be treated with cytoreductive therapy to maintain HCT < 45%, platelet count of ≤ 400 × 10
9/L, and WBC count of < 10 × 10
9/L, as proposed in current treatment guidelines [
15]. However, cytoreduction has not been shown to be effective in preventing recurrence of SVT. In a retrospective study of patients with MPNs (
n = 181), the incidence rate of recurrent events in patients treated with cytoreduction was similar to that observed in patients without cytoreductive treatment (4.2 vs 4.0 per 100 patient-years, respectively) [
84]. Overall, treatment of SVT in patients with MPNs remains an unmet clinical need, and additional studies are needed to assess potential treatments.