Application and investigation of thrombopoiesis-stimulating agents in the treatment of thrombocytopenia

THPO and THPO-RAs

IL-11-based agents

IL-11 is a cytokine which belongs to interleukin-6 (IL-6) family with hematopoietic proliferative properties. It is crucial for promoting MK colony formation and plays a stimulating effect on thrombopoiesis. ⁴¹ Various IL-11-based agents, such as recombinant human IL-11 (RhIL-11), hyper-IL-11 (H11), and PEGylated IL-11, are currently or potentially used to treat thrombocytopenia.

The recombinant form of IL-11, RhIL-11, signals through the gp130 subunit of the receptor. Co-expression of the IL-11 receptor α-chain and gp130 results in high-affinity binding, initiating signal transduction of the JAK/TYK and the MAPK. ⁴² Due to RhIL-11’s inducing MKs maturation and differentiation, and normalizing Th1/Th2 and T-bet/GATA-3 ratios close to healthy states, ⁴³ it shows therapeutic effect in ITP, CIT, and thrombocytopenia in sepsis patients.^44–46 RhIL-11 also acts as a tissue protector in anticancer therapy because it activates the gp130 signaling, JAK/TYK, and MAPK signaling, downregulates apoptotic gene BAX, upregulates anti-apoptotic gene BCL-2, and induces IEC-6 cell cycle arrest,^47,48 although the main indication of RhIL-11 is treating thrombocytopenia.

As a fusion protein comprising IL-11, H11 targets the gp130 signal-transducing subunit. ⁴⁹ Compared with rhIL-11, H11 shows more effectiveness in strengthening early progenitors’ expansion and directing them to MKs, inducing the maturation of MKs and the release of platelet-like particles (PLPs). Therefore, H11 may be more productive for stimulating the MK colony formation and PLPs production than RhIL-11, that may be beneficial for thrombocytopenia treatment and an ex vivo expansion of MKs. ⁵⁰ Compared with IL-11, PEGylated IL-11 not only has a longer half-life but also displays its ability of inducing longer-lasting increases in hematopoietic cells. ⁵¹ H11 and PEGylated IL-11 may have better potent effects on stimulating thrombopoiesis, but further clinical evidence of H11 and PEGylated IL-11 in thrombocytopenia treatment is needed compared with RhIL-11.

ATRA

ATRA is commonly used to treat skin diseases, inflammatory diseases, and acute promyelocytic leukemia (APL). ⁵² It relieves psoriasis, acne, psoriasis and ichthyosis by stimulating FOX1 gene expression,^53,54 regulates the cellular function of Th17/Treg to treat bronchial asthma, inflammatory bowel disease (IBD), periodontitis,^55–57 and activates the transcription factor FOXO3A in APL cells. ⁵⁸ Concentration of 10⁻¹² M ATRA could stimulate thrombopoiesis from hematopoietic progenitors in the presence of THPO or granulocyte-macrophage colony-stimulating factor (GM-CSF) plus IL-3. ⁵⁹ ATRA could stimulate thrombopoiesis in MEG-01 cell by promoting MK differentiation and generation of functional PLPs. These functional PLPs could be activated by thrombin receptor-activating peptide 6 (TRAP-6) with increased externalization of P-selectin. ⁶⁰ An increasing level of serum THPO was observed in APL patients with ATRA treatment, resulting in an increase in platelet counts. ⁶¹ A phase II clinical trial of 66 patients who were newly diagnosed with primary ITP reported that the proportion of patients with a sustained response (maintaining platelet counts at the level of at least 30 × 10⁹/liter and at least twice higher than the baseline counts, without bleeding events and no rescue medication being required in 6 months) in the ATRA plus high-dose dexamethasone group (68%) was significantly higher than that of high-dose dexamethasone monotherapy group (41%), representing a novel therapeutic regimen for ITP. ⁶² Compared to low-dose rituximab (LD-RTX) monotherapy, ATRA plus LD-RTX increased the overall and sustained response for corticosteroid-resistant or relapse ITP patients (80% versus 59% of overall response and 61% versus 41% of sustained response). ⁶³ These clinical investigations demonstrated ATRA might stimulate thrombopoiesis through a THPO-dependent manner, which may be beneficial in relieving thrombocytopenia, and drug combination is necessary.

Catecholamine

Catecholamines are a group of tyrosine-derived neurotransmitters, including dopamine (DA), norepinephrine (NE), and epinephrine (EPI). DA has therapeutic effects on cardiovascular diseases (hypertrophy, myocardial ischemia, hypertension, and arrhythmia) and acute renal failure. ⁶⁴ NE and EPI are commonly used to deal with severe allergic reactions, septic shock, and cardiopulmonary resuscitation.^65–67 Animal experiment demonstrated that EPI and NE in response to sympathetic stimulation could promote platelet production by the activation of α2-adrenoceptor-mediated ERK1/2 signaling, which induces MKs adhesion and migration, and the ERK1/2 activation-mediated RhoA GTPase signaling could stimulate proplatelet formation (PPF), platelet release, and platelet activation. ⁶⁸ However, the insufficient evidence of the effect of exogenous NE and EPI on platelet production requires further investigation to clarify the association between medicinal NE and EPI with thrombopoiesis. As the precursor of NE and EPI, DA cannot accelerate the early stages in thrombopoiesis proceedings but promote production of platelet from MKs in the final stages of thrombopoiesis, relating to oxidative stress-mediated signaling pathways which consist of p38 MAPK, JNK/SAPK, and caspase-3 pathways in human primary MKs. In addition, administration of DA in mice could elevate platelet counts. ⁶⁹ These findings of catecholamines on stimulating thrombopoiesis provide new insights into drug treatment of thrombocytopenia.

Estrogen

Estrogen, one of the primary sex hormones, plays various effects in relieving perimenopausal syndrome, osteoporosis, ovarian dysfunction, functional uterine bleeding, breast cancer (post-menopause), prostate cancer, and acne.^70–72 Estrogen receptors (ER), including ERα and ERβ, which express on MKs, could mediate the promoting effect of MKs differentiation, PPF, and platelet production.^73,74 Estrogen-induced platelet production may be regulated by the ERβ-dependent pathway. Through detecting BM-derived MKs and Meg-01 cells of estrogen-treated mouse, it could be found that estrogen acts as a potential activator of GATA-1, then promotes the activation of STAT1 for MKs polyploidization, maturation, and platelet production. That is consistent with the result that platelet counts in estrogen-treated mice had a significant increase. ⁷⁵ Evidence from clinical cases in postmenopausal women receiving estrogen replacement therapy showed an increase in MK numbers, ⁷⁶ but the stable effect of estrogen on elevating platelet counts in human, especially thrombocytopenia patients still require further investigation.

Melatonin

Melatonin is a hormone synthesized and secreted mainly from the pineal gland. ⁷⁷ It is mainly used as a drug to treat sleep disorders, such as insomnia and circadian rhythm disorders. ⁷⁸ It could be potentially beneficial to the treatment of neurological diseases, cardiovascular diseases, bone diseases, and cancers.^79–82 Therapeutic potential of melatonin in thrombocytopenia is under development. Melatonin could promote thrombopoiesis by increasing HSCs and MKs expansion, PPF, and platelet formation. ⁸³ From a molecular perspective, melatonin-induced thrombopoiesis might be mediated by ERK1/2 and Akt signaling. ⁸³ Besides stimulating thrombopoiesis, melatonin could inhibit MKs apoptosis by activating Akt signaling through melatonin receptors. ⁸⁴ Recent clinical evidence supported that compared with placebo, melatonin treatment could significantly enhance platelet counts (from 175.67 ± 92.84 to 191.10 ± 98.82) in patients who were affected by liver disease, reflecting its potential in thrombocytopenia treatment. ⁸⁵ Further clinical evidence on patients with different kinds of thrombocytopenia is required, and melatonin is hopeful to become novel choice in the drug treatment of thrombocytopenia.

Recombinant human growth hormone

Recombinant human growth hormone (rhGH) is used for replacing treatment of growth hormone deficiency. ⁸⁶ It also acts as the thrombopoiesis-stimulating agent which promotes various steps in platelet development, including the late stage of MK differentiation, PPF, and production of platelet. ⁸⁷ Treatment of rhGH could activate STAT5. In an early phase, activation of ERK1/2 proceeds slowly but in a prolonged period in M07e megakaryoblastic cells and primary MKs treated with rhGH. However, activation of Akt was only observed in mature MKs but not in M07e megakaryoblastic cells, implying that the terminal differentiation of MKs induced by rhGH may be attributed to the Akt pathway. ⁸⁷ Moreover, through significantly accelerating and promoting platelet production, rhGH was manifested to strengthen the effect of a tandem dimer of thrombopoietin mimetic peptide (dTMP) on thrombopoiesis in M07e cells and mice model of radiation-induced thrombocytopenia (RIT), denoting that rhGH and c-MPL may achieve thrombopoiesis complementarily and synergistically. ⁸⁷ More clinical evidence is required to confirm the promoting effect of rhGH on relieving thrombocytopenia, especially in combination with THPO and THPO-RAs.

Clozapine

Clozapine is an antipsychotic drug effective for resistant schizophrenia. ⁸⁸ However, its stimulating effects on the hematopoietic system, such as agranulocytosis, leukocytosis, and thrombocytosis, are recognized as significant side effects.^89,90 A follow-up study about clozapine’s stimulating effect on patients’ thrombopoiesis of a 1-year period demonstrated that since the beginning of clozapine treatment in the first week, platelet counts increased synchronically but transiently, and the early spike in platelet counts reflected raised IL-6 levels, which induced thrombopoiesis through the gp130 signaling and interaction with IL-3 and THPO.^91,92 This result suggests that IL-6 might be the crucial factor in clozapine-induced thrombocytosis. However, the direct evidence of clozapine’s interactions with IL-6 or other important factors such as THPO to promote thrombopoiesis is required for affirming the relevance of clozapine to the platelet development process.

Yes-associated protein inhibitors

Yes-associated protein (YAP) is an effector molecule in the Hippo pathway, which plays an essential role in MKs proliferation, differentiation, and production of platelet. ⁹³ Dobutamine and verteporfin are two Food and Drug Administration (FDA)-approved YAP inhibitors. Dobutamine is commonly used for septic shock and heart failure,^94,95 and verteporfin for cancer, age-related macular degeneration (AMD), central serous chorioretinopathy (CSC), ocular histoplasmosis syndrome, and pathologic myopia.^96–98 In vitro experiment revealed that dobutamine and verteporfin could induce megakaryocytic cell maturation and generate thrombopoiesis. ⁹⁹ Dobutamine phosphorylates YAP, inhibits YAP translocation into the nucleus, and interacts with its target genes. ⁹⁹ Verteporfin inhibits the interaction between YAP and transcriptional enhanced associate domain (TEAD) and downregulates the expression of their downstream target genes. ⁹⁹ Treatment with dobutamine or verteporfin on MEG-01 cells results in an increasing level of MKs maturation, differentiation, and production of platelets, and their thrombopoiesis-stimulating effects may be related to upregulation of Fli-1, ⁹⁹ indicating this group of agents’ potential effect in the treatment of thrombocytopenia.

Natural agents that promote platelet production