Impact of Hydroxyurea to Treat Haematological Disorders on Male Fertility: Two Case Reports and a Systematic Review

Hydroxyurea (HU) is an anti-proliferative drug synthesized by Dresler and Stein in 1869 and firstly used as an anti-tumor agent in the 1960s in the setting of DNA synthesis through inhibition of the ribonucleotide reductase enzyme [1, 2]. HU is currently used as first-line therapy of sickle cell anaemia/disease (SCD) [3, 4, 5], essential thrombocythemia (ET) [6, 7, 8], polycythaemia vera (PV) [7, 9], leukemia [10, 11], psoriasis [12], and HIV infection [13] and in combination with radio-chemotherapy for some malignancies [14, 15]. The primary site of action of HU is the ribonucleotide reductase, also known as ribonucleoside diphosphate reductase, a crucial enzyme in DNA synthesis which acts by catalyzing the reduction of ribonucleotides to deoxyribonucleotides [16]. Its inhibition leads to the depletion of intracellular deoxynucleotide pools, to the impairment of DNA synthesis and repair, and to block cell mitosis in the S-phase [1]. Moreover, HU causes cell apoptosis by directly cleaving DNA molecules, breaking chromosomes in the metaphase’s mitotic division, and, once converted to hydroxylamine, by cleaving acetyl-coenzyme A and disrupting oxidative phosphorylation [17]. HU related cytotoxicity relies both on its concentration and length of treatment, whilst the effects are usually reversible at drug discontinuation [18]. As a consequence of its pharmacodynamics, common adverse events include bone marrow suppression, bleeding, headache, dizziness, fatigue, various cutaneous disorders, nausea/vomiting, gastric pain, and diarrhea [19, 20, 21].

SCD is the most diffuse inherited hemoglobinopathy worldwide [22]. It is characterized by the production of hemoglobin S, which, when deoxygenated, crystallize and alter the structure of red blood cells giving the characteristic sickle shape [23]. The loss of the normal biconcave shape induces acute or chronic vasa-occlusion, haemolytic anaemia and vasculopathy with painful crises, sometimes accompanied even by acute chest syndrome, stroke, organ failure and eventually death [24]. SCD is a life-long process, with daily pain symptoms starting throughout childhood [25], and increasing over adolescence into adulthood [26].

ET is a Ph-negative chronic myeloproliferative tumor characterized by increased production of functionally poor platelets [7, 27, 28]. The clinical course of the disease is complicated by hemorrhagic and, more frequently, thrombotic events [29], with an overall dysfunction of the microcircular system [30]. Major risk associated with ET is the evolution to neoplasm and patients below 40 years of age seem to be at higher risk [31].

PV is a chronic clonal myeloproliferative tumor characterized by increased production of red cells with high levels of hemoglobin [32, 33]. Patients experiment fatigue, pruritus, and splenomegaly together with an increased risk of thrombotic and cardiovascular events [7, 34]. Despite being more common among the older, it can occur also in men younger than 20 years of age [35].

These three hematological diseases can be treated with HU, which should be offered as soon as possible after the diagnosis, starting from 9 months of age [36, 37]. As these patients get older, one of the problems they may encounter is the impairment of spermatogenesis due to a decreased spermatogonial maturation, resulting in worsening of sperm parameters till azoospermia and eventually fertility issues [38, 39].

Consequently, on the one hand considering the need for a relatively secure treatment for myeloproliferative diseases (ET and PV) and hemoglobinopathy (SCD), and, on the other, the willingness of fatherhood, current systematic review aims at summarizing the published evidence on the impact of HU toward male fertility. Likewise, to support published literature and provide further insight over the management work-up of men presenting for couple’s infertility, we also present two clinical cases of patients under HU treatment because of ET and PV, respectively, who had been referred to our tertiary referral reproductive medicine center. The final aim of this work is to provide practical advice and guide andrologists through an effective clinical work-up and decision-making in this relatively neglected field.

We conducted a systematic review of all published articles related to the effects of HU in terms of male fertility. PubMed, EMBASE, and Cochrane were queried for all the published studies indexed up to March 2022. A combination of the following keywords was used: “hydroxyurea”; “fertility”; “male”; “sperm”; "sickle cell anaemia”; “sickle cell disease”; “essential thrombocythemia”; “polycythaemia vera”. Only original articles that included patients affected by SCD, ET, and PV, treated with HU as first-line therapy, and whose fertility was evaluated by means of sperm parameters or testicular tissue analysis before (i.e., naïve patients), during, or after HU treatment, were considered for inclusion in the current systematic review. Titles and abstracts of manuscripts were screened for initial study inclusion. Full text review was performed when the abstract was not sufficient to determine study inclusion. Abstracts, commentaries, editorials, articles that did not undergo peer-review, and studies on animal models were excluded. References lists of included studies were hand-searched for completeness. Full text review was performed when the abstract was not sufficient to determine study inclusion. In the case of multiple publications from the same cohort, the most updated one has been included. Finally, non-English studies, those without an evaluation of patients’ fertility with semen analysis or testis specimen, and studies where different treatment modalities were used for SCD, ET or PV, were also excluded from the current systematic review.

This systematic review was conducted based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and registered to PROSPERO with ID number CRD42022375767. Two authors completed the study selection independently (S.C. and G.F.) according to PRISMA requirements. Potential disagreements were resolved by consensus among all co-authors. The risk of bias was assessed by use of the Risk Of Bias In Non-randomized Studies of Interventions (ROBINS-I).

Overall, 48 articles have been identified; thereof, 40 articles were excluded (i.e., 19 duplicates; 12 because only the abstract was available; 3 dealt with other diseases; 5 were pre-clinical/animal studies; and 1 was a review). Of all, 8 manuscripts accomplished our inclusion criteria and have been considered for this systematic review (Fig. 1). Risk of bias according to the ROBINS-I tool emerged to be severe in 3, moderate in 3 and low in 2 studies, respectively (Supplement Table 1).

Fig. 1
Prisma flow chart—study selection with inclusion and exclusion criteria of reviewed studies.

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Table 1 summarizes the characteristics of the included studies. Overall, 161 patients were included in the current systematic review. The oldest study was published in 2007 and the most updated in 2021. Of all, 6 studies investigated SCD, 1 study both SCD and PV, and 1 ET alone. Overall, median age of patients ranged from 7.9 to 38 years and the length of HU administration from 1.4 to 10.5 years. As a whole, male fertility was investigated with semen analysis in 5 studies (Table 2), and in 3 studies with testicular tissues cryopreserved before cytoreductive therapies; of these, 2 studies reported the number of spermatogonia per round cross section of seminiferous tubule (S/T ratio), and 1 study reported spermatogonia’s maturation (Table 3).

Table 1
Characteristics of the included studies classified according to year of publication (2021–2007)

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Table 2
Semen parameters reported in analysed studies classified according to year of publication (2007–2021)

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Table 3
Tissue specimen data in analysed studies classified according to year of publication (2021–2007).

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5. Cases reports

1) A case of secondary couple’s infertility with a diagnosis of polycythaemia vera under treatment with hydroxyurea

We present the case of a 40-year-old man who was referred to our center for secondary couple’s infertility (the first child was 2.5 years old at patient presentation). The patient was diagnosed with myeloproliferative disease PV JAK2V617F+ according to WHO 2016 classification [49, 50]. The diagnosis was performed in concomitance with diagnosis of Multiple Sclerosis during the hospitalization due to thrombosis of the left transverse sinus in 2020. The patient was classified as high vascular risk according to ELN and started treatment with HU in combination with antiplatelet therapy [51].

Trying to conceive, the patient presented in our fertility clinic with a semen analysis reporting cryptozoospermia (i.e., 1 sperm per 25 High Resolution Fields). The only treatments ongoing at time of fertility evaluation were HU for PV and dimenthyl fumarate for Multiple Sclerosis.

His wife, followed at the same ART center, had no fertility issues after a comprehensive Gynecological evaluation [52].

As for each infertile male patient, a complete medical assessment with detailed medical and reproductive history taking and physical examination was performed, which revealed no additional medical issues. Blood test with complete hormonal profile was carried out without finding no additional issues. Ultrasound scan of the testis and of the lower abdomen revealed no morphologic abnormality in the reproductive system. As such, the most probable etiology of secondary infertility was attributed to the initiation of HU. Thereafter, HU treatment was discontinued in accordance with the treating Hematologist in January 2022, and a bloodletting regime was started. After 5 months from HU discontinuation, a spontaneous pregnancy did occur; moreover, at semen analysis performed 6 months after HU was stopped a significant improvement of all semen parameters was achieved up to normozoospermia (Table 4) [53].

Table 4
Semen parameters of the two case reports included in our study

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2) A case of primary infertility with a diagnosis of essential thrombocythemia under treatment with hydroxyurea

We present the case of a 34-year-old man who had received six years before a diagnosis of ET according to 2008 WHO classification, with platelets count over 1,000×10⁹/L, an increased megakaryocytes count at bone marrow biopsy, without driver mutation JAK2V617F–CALR–MPL negative) also confirmed at NGS study [54]. The patient was classified as high risk due to platelets count above 1.500×10⁹/L and started HU according to ELN 2011 [51]. After 5 years of HU therapy, the patient was referred to our center because of primary couple’s infertility and the finding of azoospermia at semen analysis, while first level investigations including physical examination, ultrasound assessment of the testes and the abdomen and circulating hormones were normal. Past medical history revealed the presence of Gilbert syndrome and a cholecystectomy, which were however not related to his current infertility problem. The partner, followed at the same ART center, had no fertility issues after a comprehensive Gynecological evaluation [52]. In the hypothesis that HU was the cause of his azoospermia, HU treatment was discontinued in accordance with the treating Hematologist and the patient did start salicylic acetyl acid treatment. After 3 months, a partial improvement in sperm concentration (6.0×10⁶/mL) was depicted at semen analysis, with a diagnosis of oligoasthenoteratozoospermia. The further semen analyses at 5-, 6-, and 8-month investigation from HU discontinuation, confirmed the observed improvement in terms of sperm concentration (the last 7.5×10⁶/mL) (Table 4). A full-term pregnancy was achieved after two years form the drug suspension, while patient was under salicylic acetyl acid treatment.

This systematic review of the literature provides findings from 8 studies on the impact of HU treatment in men with SCD, PV, or ET toward male fertility outcomes. Of these, only two case reports dealt with infertility issues in patients with either PV or ET. Overall, evidence on infertility in patients treated with HU are scant and definitive conclusions cannot be drawn. To sum published literature up, in prepubertal patients with SCD, testes quality seemed similar between treated and untreated patients, despite being lower compared to healthy individuals. In adult patients with SCD several sperm abnormalities have been reported already before therapy with HU, with a further worsening of semen parameters during ongoing treatment with HU, and without consistent evidence of improvement after drug discontinuation. Moreover, the analysis of testicular sample revealed no differences in spermatogonial density between HU-treated and healthy tissues—although potentially biased since derived from orchiectomy performed in patients with testicular cancer—despite a decreased spermatogonial maturation. Conversely, azoospermia was more clearly reported to occur during HU therapy in men with ET or PV, and normozoospermia was eventually restored after treatment discontinuation. The length of HU discontinuation before restoration or at least improvement in semen parameters ranged between 1 month and 13 years. Similarly, here we presented the case of one patient with PV and of one patient with ET, both treated with HU and seeking medical help at our andrology center for couple’s infertility. Of clinical relevance, both patients had a significant improvement of all semen parameters after HU discontinuation, at 3 and 6 months, respectively. Moreover, both patients eventually spontaneously fathered.

These data deserve further considerations.

First, there is evidence that SCD itself might cause an impairment of spermatogenesis. In fact, our review found that HU-naïve patients diagnosed with SCD have worse quantitative and qualitative semen abnormalities compared to healthy individuals [5, 36, 41, 43, 47]. Gonadal dysfunction due to SCD could results from vasa-occlusion phenomenon of either hypothalamic-pituitary blood vessels, with the result of secondary hypogonadism, or of the testicular vessels, thus resulting in organ failure and primary hypogonadism [55]. Indeed, the hypoxic-ischemic damage of testicular tissues in men with SCD creates alterations similar to the ones observed in testicular tissues in men with varicocele, postulating that hypoxia is a contributing factor to sperm abnormalities in these patients [56, 57].

Second, literature is poor on this issue despite SCD, PV and ET are quite frequent diseases and HU represents often their first-line treatment. Of note, HU is indicated as primary therapy in patients affected by PV and ET classified as high risk, and also in low-risk patient with at least one among adverse features including leucocytosis, poor hematocrit control from phlebotomy-only, symptomatic progressive splenomegaly, and in patients with progressive and persistent leucocytosis, extreme thrombocytosis and inadequate hematocrit control from phlebotomy-only (>6/year), as it has been highlighted by the ELN 2021 recommendation [58]. This will potentially increase the number of young patients candidate to cytoreductive treatment unveiling the need for a fertility counseling and management. Infertility should be considered among the issues relevant for the selection of a second line treatment in high-risk patients according to ELN 2021; thereof, further data are needed to corroborate this hypothesis.

Third, published data regarding HU discontinuation as a potential modality to improve semen parameters, supporting the post-hoc hypothesis that treatment with HU might cause an impairment in spermatogenesis, are conflicting. Indeed, animal studies have shown that HU in mice increases testicular germ cell apoptosis and reduces spermatogenesis [37]. Results from our systematic review suggested that HU could worsen the testicular function in patients with hematological diseases. Following this reasoning, HU discontinuation has the potential of improving spermatogenesis, especially for patients with ET and PV. Of note, data are much more conflicting for SCD, since the only three studies which had analyzed the impact of HU in this setting, both during HU treatment and after HU discontinuation, showed that improvement was not present at drug discontinuation, regardless of the timing from HU suspension.

Overall, males with SCD might suffer from semen abnormalities that are also independent from the HU itself and therefore not reversible after HU discontinuation. Conversely, for PV and ET, the two published case reports and the two clinical cases presented here are concordant in supporting HU discontinuation to achieve better semen parameters and eventually even pregnancy and live births. This strategy should be discussed with the treating hematologist.

The present manuscript and the findings of our systematic review are certainly not devoid of limitations. First, the small number of studies published on this issue, the small sample size of these studies, their high heterogeneity in both patients’ characteristics and HU treatment duration and time frame since discontinuation, prevent to formulate definitive recommendations on the management of infertility in case of treatment with HU in patients with SCD, PV and ET. In this context, the very few studies published on this topic have involved a very small number of patients and were all retrospective in nature, thus implying a high risk of relevant biases. In addition, there is a high heterogeneity in the study populations, follow-ups and outcome definitions. Moreover, most of the selected studies had a “moderate risk” of bias according to the ROBINS-I tool. Overall, these issues largely prevented us from drawing definitive recommendations regarding the use of HU in patients affected by hematological diseases. Finally, the biological underpinnings of these findings are not known, and achieving an adequate explanation is key to justify any clinical decision making in this delicate setting.

Overall, with the specific aim of both preserving and restoring fertility potentials in male patients who wish to fathering but who require HU for their relevant hematological disorders, further research is needed to i) better understand testicular and accessory glands alterations in SCD patients, ii) depict the actual effects of HU discontinuation, and iii) find new strategies, together with the hematological team.

In conclusion, considering the several limitations of this current systematic review and recognizing the rarity of these conditions, no definitive conclusion could be drawn especially in men with SCD, and further studies are needed. However, a comprehensive discussion in a multidisciplinary team both involving the andrologist and the hematologist is of utmost importance when dealing with male infertility. It is likely that HU has a detrimental effect on spermatogenesis, due to its pharmacodynamic profile and irrespective of the baseline disease. In addition, in men with PV and ET, HU discontinuation might significantly improve semen parameters and increase pregnancy probability, whereas evidence in case of SCD is more conflicting, probably because SCD per se might cause irreversible damage to the testis and hence to the spermatogenesis.

Supplementary materials can be found via https://doi.org/10.5534/wjmh.230069.

Supplement Table 1

ROBINS-I assessing the risk of bias in studies included in the systematic review, classified according to parameters evaluated and year of publication