Acute myeloid leukaemia (AML) is a clinically and biologically heterogeneous disease characterized by the accumulation of immature transformed myeloid progenitors in bone marrow. Although huge step forward have been achieved in improving outcomes for AML patients, the standard therapy for most subtypes of newly diagnosed AML has remained practically unchanged over the past 40 years, and the prognosis remains poor. Indeed, most patients with AML will relapse after achieving complete remission, with treatment of refractory and relapsed AML being challenging in clinics. An initial great enthusiasm came from the discovery that retinoic acid and its derivative were able to destroy AML cells bearing the Bcr-Abl chimaera responsible for their oncogenic potential. A further milestone step was done by the discovery of imatinib, which represent the firstly clinically approved targeted therapy to treat AML and then other cancers. Soon, however, scientists realized that the Bcr-Abl oncogene was able to originate mutants that would resist to imatinib, forcing researchers either to develop better derivatives and to screen for other signaling and metabolic pathway worthy being target to.
It is widely known that neoplastic cells show significant changes in glucose metabolism relying on glycolytic and pentose phosphate pathways instead of oxidative phosphorylation for energy production. This fact leads to acidic species overproduction and therefore a strong need for proton extrusion to avoid intracellular acidification. In this sense, several proton transporters were found overexpressed in tumor cells and they have been a matter of study in recent years. Monocarboxylate transporters (MCTs), sodium proton exchangers (NHEs) and V-ATPase are the structures that most captured the attention in this field. There is also the voltage-gated proton channels (Hv1), which has a strong capacity of restoring intracellular pH after heavy acid loads; and as it is a passive transport pathway, its activity does not require any expense of energiy in form of ATP, which is very convenient for cancer energetic currency spare. This membrane channel si upregulated in leukemia cells as well and seem to promote neoplastic progression. Its activity can be antagonized by zinc ions (Zn2+) but among known drugs there is an antihistamine called diphenhydramine.
This very old drug is perhaps the prototype of the first generation of antihistamine drugs acting on H1 receptors. It has antillergic, sedative, antivomiting and antitussive properties. However, its is very unspecific and it holds collateral effects, because beside H1 receptor this drug also may bind to muscarinic subtypes and cortical NMDA receptors that may explain most of its side effect (mostly drowsiness, sedation and dry mouth). Just a couple of years ago, this drug has been tested against Jurkat T cell leukemia lines, which express the Hv1 channel. Upon exposure, malignant cells time-dependently developed anomalies in their volume and in the acid content in the cytosolic environment. By inhibiting active proton currents, diphenhydramine triggered the onset of apoptosis similarly to that started by an excess of zinc ions. However, the drug may possess additional cellular targets, since in the context of B16-F10 melanoma cells, it was able to interfere with the activation of STAT-3, irrespective of a B-RafV600E mutation. This indicate that the drug could not be a protein kinase, yet it affects a transcription factor like STAT-3 which claims an upstream kinase cascade for its activation. But mechanism have not been delved into.
Lysosomes in chronic lymphocytic leukemia (CLL) cells have previously been identified as a promising target for therapeutic intervention in combination with targeted therapies. Recent studies have shown that antihistamines can induce lysosomal membrane permeabilization (LMP) in a variety of cell lines. Moreover, data indicates that lysosomotropic agents can cause synergistic cell death in vitro when combined with some tyrosine kinase inhibitors (TKI). Among these there are three over-the-counter antihistamines, desloratadine, loratadine and clemastine, that may induce CLL cell death via lysosome involvement. Among these, clemastine was the most effective at inducing LMP and cell death in laboratory models. More importantly, the antihistamines induced synergistic cytotoxicity when combined with the tyrosine kinase inhibitor, ibrutinib, but not with other chemotherapy drugs. Together clemastine and ibrutinib induced reactive oxygen species (ROS), loss of mitochondrial membrane potential and decreased the expression of the protective protein Mcl-1, leading to apoptosis.
Importantly, studies seem to indicate that clemastine exert its effect without intereacting with histamine receptors, therefore without altering intracellular messengers like cyclic nucleotides (cAMP or cGMP) or lipides like diacylglicerol (DAG) or other phospholipids. This would imply that leukemia cells do not express any type of histamine receptor but is not indeed the case. Nevertheless, antihistamines appear to work like they are not seeing their cognate docking site on histamine receptors. Another example for this principle is represented by ciproheptadine, a first-generation anti-H1 and employed against eczema, hay fever and atopic dermatitis. In mouse models of myeloma and leukemia, cyproheptadine inhibited tumor growth without significant toxicity. Ciproheptadine-induced apoptosis was preceded by activation of the mitochondrial pathway of caspase activation and was independent of the drug’s known activity as an H1 histamine and serotonin receptor antagonist. Its mechanism has been found to rely on the inhibition of histone deacetylation. As an histone deacetylase (HDAC-1) inhibitor, cyproheptadine is structurally different form other known and clinically approved HDAC inhibitors.
In myeloma and leukemia cells obtained form patient samples, it decreased expression of cyclins D1, D2, and D3, arrested these cells in the G0/G1 phase and finally induced programmed cell death, without affecting normal hematopoietic cells. Beside, its has another potential target which is the PI3K-Akt axis, which drives cellular proliferation and escape from programmed cell death. Both in leukemia and myeloma cells, PI3K/Akt signaling pathway is central for cellular survival and clonal expansion even in absence of serum, cytokines or growth factors usually required from these cells (e.g. IL-6). In addition, like clemastine plus ibrutinib, diphenhydramine shows synergic effect with bortezomib, a well-approved standard chemotherapic in myeloma management. Thus, cyproheptadine represents a lead for a novel therapeutic agent for the treatment of blood malignancies. Because the drug is well tolerated and already approved in multiple countries for clinical use as an antihistamine and appetite stimulant (Periactin), it could be moved directly into clinical trials for cancer.
But it is not the only one. Being already FDA-approved allergy medications, these molecules do not need a de nove investigation and they are alredy known to be non toxic. For example, astemizole (Hismanal) and terfenadine (Seldane) are widely prescribed for allergy management. These compounds are both inverse agonists of the histamine receptor H1and both negatively affected all the leukemic populations equally in the micromolar range and they do not generally induce drowsiness. Neverthelss, again, their activity is not dependent on surface receptors since histamine is not able to rescue the killing effect on leukemia cells. Drug bioscreening revealed that they both may block the enzyme acid sphyngomyelinase (ASMase). ASMase is appointed to drive sphingolipid metabolism inside cells. It cleaves membrane ceramide into the sphingosine and O-phoshocoline bases and is genetically coded in two isoforms: an extracellular and an intracellular. The latter gets anchored in lysosomes. Therefore, one may speculate that antihistamines, as discussed before, have this lysosomotropic properties because they target sphingolipid metabolism in these organelle.
However, ASMase activation rather than inhibition is the mechanism that has been invoked for leukemia cell death induced by anticancer drugs, radiation and oxidative stress. Sphyngomyelin breakdown by ASMase originates ceramide, which is well documented to trigger programmed cell death in cancer cells as well as in many other non-oncogenic contextes. Though alternative sphingolipid metabolism may be hypothized, antihistamines are essentially nitrogen bases and thy are hydrophobic but with a positively charged tail. So they are not ionized at a physiologic pH and since they have overall hydrophobicity, they can easily permeate through the plasma membrane. Within the highly acidic lysosomal compartment, the amine is protonated, the drug is then trapped and concentrated, inducing membrane structure perturbation and compromising cell viability. Considering the other ASMase antihistamine inhibitor mentioned, astemizole, it has in addition a further ability: it may inhibit the mutated form of JAK2 protein kinase (V617F), an oncogenic form that clinically drives the progression of myelodysplastic sindromes (pre-leukemia) and other bone marrow neoplasms.
Therefore, beside, being a lysosome-perturbing compound, it might interfere at relevant doses also with intracellular tyrosine kinase pathways implicated in conditions like idiopathic myelofibrosis, polycitemia vera and essential thrombocytopenia. In this regard, these conditions have classically being treated in the past with cytotoxic agents like ciclofosfamide, busulfan, corticosteoids and other immunosuppressants. All of these choices not only did put patients at higher risk for carcinogenic transformation of the conditions themselves, but they did not actually changed prognosis that much. A more modern approach has come with the development of specific either JAK2 and JAK2V617F inhibitors like tofacitinib and ruxolitinib, which improved the overall survival while having a much more contained side effects. Astemizole could represent and advantegous compound bearing either the lysosome-destabilizing effect and the ability to interfere with tyrosine kinase signaling in leukemogenic cells. It may help develop a most easily configurable combinatorial protocol to treat blood malignancy without the worrisome side of the toxicity.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
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