Drug Candidates Targeting HTLV-1 and Related Diseases: History
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Contributor:
Milena Cristina Martins da Silva
,
Renan Stefferson Barradas Pereira
,
Antonia Cherlly Aparecida Araujo
,
Ednilson Gregorio da Silva Filho
,
Anderson de Lima Dias
,
Kassio Silva Cavalcante
,
Maísa Silva de Sousa

Among the human T-lymphotropic virus (HTLV) types, HTLV-1 is the most prevalent, and it has been linked to a spectrum of diseases, including HAM/TSP, ATLL, and hyperinfection syndrome or disseminated strongyloidiasis. There is no globally standard first-line treatment for HTLV-1 infection and its related diseases. To address this, a comprehensive research was conducted, analyzing 30 recent papers from databases PubMed, CAPES journals, and the Virtual Health Library (VHL). The studies encompassed a wide range of therapeutic approaches, including antiretrovirals, immunomodulators, antineoplastics, amino acids, antiparasitics, and even natural products and plant extracts. Notably, the category with the highest number of articles was related to drugs for the treatment of ATLL. Studies employing mogamulizumab as a new perspective for ATLL received greater attention in the last 5 years, demonstrating efficacy, safe use in the elderly, significant antitumor activity, and increased survival time for refractory patients. Concerning HAM/TSP, despite corticosteroid being recommended, a more randomized clinical trial is needed to support treatment other than corticoids. The research also included a comprehensive review of the drugs used to treat disseminated strongyloidiasis in co-infection with HTLV-1, including their administration form, in order to emphasize gaps and facilitate the development of other studies aiming at better-directed methodologies. Additionally, docking molecules and computer simulations show promise in identifying novel therapeutic targets and repurposing existing drugs. These advances are crucial in developing more effective and targeted treatments against HTLV-1 and its related diseases.

  • HTLV-1-related diseases
  • drug treatment
  • ATLL
  • HAM/TSP
  • molecular docking

1. Introduction

The human T-lymphotropic virus (HTLV) belonging to the Retroviridae family was discovered and isolated for the first time in 1980, and since then [1], it has been the subject of scientific discussions due to its relationship with other infections and clinical conditions since it has the capacity to infect the cells of the immune system, reducing the body’s defense [2].
Currently, four types of HTLV are known: 1, 2, 3, and 4. Among these, types 1 and 2 are the most studied because they cause the highest number of infections globally [3]. HTLV-1 affects approximately 15 to 20 million individuals, with the highest rates coming from Japan, sub-Saharan Africa, South America, and the Caribbean. It affects both men and women, with a higher seroprevalence in people over 50 years of age, mainly females.
HTLV-2 affects about 800,000 individuals, with the majority in the Americas, mainly in the United States (between 400 and 500 thousand) and Brazil (between 200 and 250 thousand), but it can also be found in Europe and Central Africa. HTLV-2 is often detected in Native American populations and among IV drug users [3][4].
Types 1 and 2 share characteristics: both rely on cell–cell contact for transmission and use envelope glycoproteins to mediate attachment and entry into other cells. In addition, they use neuropilin-1 (NRP1) and glucose transporter 1 (GLUT1) receptors to bind and enter cells, respectively. However, they differ in the actions of regulatory proteins Tax-1 (HTLV-1) and Tax-2 (HTLV-2) and products of antisense genes HBZ (type 1) and APH-2 (type 2), leading to different clinical manifestations. Another difference observed is regarding cell tropism; HTLV-1 has tropism for CD4+ T cells, while type 2 has tropism for CD8+ T cells. Most HTLV-2 carriers are asymptomatic, but some neurological conditions such as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) may occur [3][4][5].
The discovery of HTLV-3 was reported by researchers in 2005 in two asymptomatic individuals living in the rainforest area of South Cameroon [6][7]. More recently, the same teams reported the discovery of two additional HTLV-3 strains in other individuals from Cameroon [8][9]. The fourth HTLV type (HTLV-4) consists of a unique human strain found in the PBMCs obtained from a hunter living in Cameroon [7].
Among the types of HTLV, the most frequent is type 1 [4][5][6][7][8][9][10]. The transmission occurs mostly through breastfeeding in infancy or sexual intercourse in adults, or through contaminated blood products and tissue transplantation [11]. Although most HTLV-1 carriers remain asymptomatic for life, part of them will develop related diseases. Several serious diseases are thought to be caused by or strongly associated with the virus. These diseases show no specific symptoms; therefore, unless HTLV-1 is considered first and serology ordered, the correct diagnosis is not possible [2].
The main diseases related to HTLV-1 infection include HAM/TSP, adult T-cell leukaemia/lymphoma (ATLL), and hyperinfection syndrome or disseminated strongyloidiasis. However, other conditions such as HTLV-1-associated uveitis (HAU), infective dermatitis, bronchiectasis, bronchitis and bronchiolitis, seborrheic dermatitis, Sjögren’s syndrome, rheumatoid arthritis, fibromyalgia, and ulcerative colitis also have been related to this virus infection [12].
Although the mechanism of HTLV-1 pathogenesis is not fully understood, it is known that the virus infects dendritic cells, macrophages, monocytes, CD8+ T lymphocytes, and mainly CD4+ T lymphocytes [13]. The mechanisms of HTLV-1 interaction with the host, host responses, and its immunogenetic characteristics seem to be the most important variables for the pathogenesis of related diseases. Nevertheless, it is still unknown why some people develop severe cases of HAM or ATLL, others have moderate disease, and many others are asymptomatic [14].
The management of patients living with HTLV-1, as well as the treatment of diseases associated with the virus, remains a challenge worldwide. Although studies about the virus and associated diseases treatment have been developed since the 1990s [15][16], some therapeutic agents used to treat its associated disease, such as ATLL, are not universally available. Zidovudine and interferon-alpha (AZT/IFN) have been used to treat ATLL in different parts of the world, including countries in Europe and America. In Japan, where several studies about the theme have been conducted [17], regimens other than AZT/IFN are used to treat ATLL. Mogamulizumab and certain components of the vincristine, cyclophosphamide, doxorubicin, and prednisone (VCAP); doxorubicin, ranimustine, and prednisone (AMP); and vindesine, etoposide, carboplatin, and prednisone (VECP) chemotherapy regimen (modified LSG15) are combined to treat ATLL in this country, but some of these drugs are unavailable in other countries, resulting in different treatment strategies around the world [18].
Despite experts mostly agreeing treatments listed are appropriate, the International Consensus report review for ATLL has the objective of benefiting patient care through recommended good practice by consensus. It particularly highlights the following points: the mechanism of action of some drugs is not completely understood; there are patients who are resistant to therapy (e.g., with AZT/IFN); several advances in the clinical management of patients with ATLL, particularly in Japan with the use of drugs such as lenalidomide, have been made; and for ensuring the consensus is continually updated to establish evidence-based practice guidelines, more studies, including clinical trials, are needed [18].
The international consensus guidelines for the management of HAM, an important HTLV-1 associated disease, describes that pharmacological treatment aims to modify the disease progression, reducing symptoms and increasing mobility. Despite there being evidence to support the use of corticoids methylprednisolone and prednisolone for progressive disease dependent on the disease stage and patients’ condition, the evidence base for guiding treatment for patients with HAM/TSP is extremely limited, mainly for therapies other than corticoids, and according to this consensus, higher-quality evidence to support any recommendations is an urgent need because the potential effect of the current (and future) therapies is uncertain [19].

2. New Studies about Drugs to Treat HTLV-1 Infection

The mechanism of HTLV-1 pathogenesis is still not fully understood. Among all the regulatory proteins encoded by proviral DNA, the Tax and HBZ proteins seem to be essential for viral pathogenesis, possibly through the induction of cell growth [20]. For this reason, they are often studied as a target for drugs. However, due to the lack of complete elucidation of the defense mechanism, the treatment is still uncertain, and studies have been developed to discover new drugs and/or determine which would be the most appropriate to be used singly or in combination as the first choice to treat patients with HTLV-1.
In this context, the potential of 1,2,3-triazole derivatives, obtained and purified by flash chromatography, was assessed by cell-based assay using the resazurin reduction method and evaluated towards cell cycle, as well as in terms of apoptosis and Tax/GFP expression analyses through flow cytometry. From the screening of 25 compounds, three active and non-cytotoxic compounds were found. They could decrease the metabolic activity of the HTLV-1-infected cell line (MT-2), promoting the significant activation of effector caspase-3/7 compared to the control, and/or interfering in LTR transactivation and/or Tax expression. The compounds can potentially contribute as a drug for ATLL patients [21].
Other synthetic compounds tested were the ones belonging to mesoionic class. The (E)-3-phenyl-5-(phenylamino)-2-styryl-1,3,4-thiadiazol-3-iuchloride derivatives were evaluated against MT-2 and C92 cell lines infected with human HTLV-1 and non-infected cells (Jurkat). The compounds also had their cytotoxicity assessed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays. The results showed IC50 values of all compounds in the range of 1.51–7.70 μM in HTLV-1-infected and non-infected cells. One of the compounds induced necrosis after 24 h in Jurkat and MT-2 cell lines. The experimental (fluorimetric method) and theoretical (molecular docking) results suggested that the mechanism of action could be related to its capacity to intercalate into DNA, and they presented spontaneous and moderate interaction with albumin in human serum albumin (HSA)-binding affinity assay by multiple spectroscopic techniques (circular dichroism, steady-state and time-resolved fluorescence), zeta potential, and molecular docking calculations, indicating good biodistribution in the human bloodstream [22].
A drug that has been studied is pomalidomide (pom). A group of researchers who had already tested this drug in vitro and proved it increased the susceptibility of HTLV-1-infected cells to NK and CTL killing used the rhesus macaque model to determine if pom treatment of infected individuals activates the host immune system and allows for the recognition and clearance of HTLV-1-infected cells. The results suggested pomalidomide can enhance the immune response to HTLV-1 infection, but this response is not maintained, and the authors suggested a combination with other well-tolerated drugs such as lenalidomide [23].
Considering the emergence of drug-resistant viruses, as well as the low efficacy, high cost, toxicity, and low bioavailability of the current antiviral drugs, returning to traditional herbal medicine is considered an alternative approach. Several medicinal plants and natural products have been identified as possible alternatives to fight HTLV-1 infection and its related diseases. For example, curcumin and its analogues have shown significant effects against retroviruses [24]. Bidens pilosa extracts have demonstrated growth suppressive effects on HTLV-1-infected T-cell lines and ATLL cells [25]. Green tea catechins, especially epigallocatechin-3-gallate (EGCG), have shown antiviral effects against HTLV-1, achieved by suppressing HTLV-I pX and Tax gene expression [26].
Knowing that many drugs were discovered and extracted from plant material, plants can be an excellent resource for the discovery of new treatments. The Eucalyptus camaldulensis (Ec) alcoholic extract was evaluated for this purpose by testing its influence on the Tax-induced activity of NF-ĸB and HTLV-1 LTR in Jurkat cells. The results showed that Ec inhibited Tax-induced activation of NF-ĸB, SRF-dependent promoters, and HTLV-1 LTR. The 40%-MeOH fraction of this extract, rich with polyphenols, offered the highest inhibitory effect against Tax activities, but further studies are required to isolate the active component/s in this extract [27].
A new perspective is the development of vaccines based on peptides, an alternative that can contain different epitopes in just one dose. One study, observing the activation of CD8+ T lymphocytes and the majority presence (88%) of the HLA-A2 and A24 alleles in HTLV-I infected individuals, conducted an immunoproteomic analysis to identify MHC-I-restricted epitopes capable of binding to these alleles and eliciting an anti-HTLV-I response by polyclonal T cells. Using the MT-2 and SLB-1 cell lines (HLA-A2+ and A24+), 6 epitopes were identified (IIN, ITN, PLL, FTD, FLN, and LFA), all originating from proteins whose actions are essential for HTLV-I pathogenesis. In vitro assays showed that CD8+ T cells activated by these epitopes secrete cytokines with cytotoxic and antiviral activity (such as IFN-γ and TNF-α). In in vivo assays, immunogenicity was confirmed by ELISpot, CD107 degranulation assay, and MagPix MILLIPLEX analysis [28].

3. Updates in Drug Research for the Treatment of Adult T-Cell Leukemia/Lymphoma (ATLL)

The adult T-cell leukemia/lymphoma (ATLL) is an aggressive lymphoproliferative disease of mature T cells, etiologically related with the human T-lymphotropic virus type 1 (HTLV-1). It has various clinical features, as well as risk factors such as maternal transmission, older age, increased proviral load in peripheral blood, and family history of ATLL that have already been identified [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30].
Although the clinical evolution of the types of ATLL is different, the more aggressive forms have an unfavorable prognosis resulting in very short overall survival (OS) [14]. In recent years, many drugs have been tested against ATLL to develop a more effective therapeutic regimen; however, there is no worldwide standard use of drugs to treat ATLL due to the heterogeneity of available drugs in countries with high prevalence of the disease and scarcity of clinical trials that investigate these drugs among first-line treatment options. The international consensus regarding the treatment of ATLL uses GRADE criteria for the quality of evidence and states that the level of evidence for ATLL should be regarded as low or very low, equivalent to a GRADE evidence score of C or D [18].
In Japan, one of the areas with the highest occurrence of ATLL, chemotherapy is used with the association of several drugs such as Hiper-CVAD (cyclophosphamide, vincristine, doxorubicin, and dexamethasone, alternating with high doses of methotrexate and cytarabine) and CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone) or CHOEP (CHOP + etoposide), but some of these drugs are not available outside Japan [31]. Antiviral therapy with zidovudine (AZT) and interferon-α (IFN) has been frequently applied for ATLL in Europe and the USA, but the mechanism of action of AZT/IFN has been partially elucidated [32]. Therefore, studies about the safety and efficacy of new therapies, as well as existing ones, are essential to guarantee the treatment of patients. The research evaluated articles produced in the last five years to verify the knowledge generated on this topic.
Regarding the treatment schemes already existing, some studies have compared them and/or proposed alterations. A retrospective study of 103 untreated aggressive ATLL patients analyzed a modified EPOCH Regimen (mEPOCH; etoposide, vincristine, doxorubicin, carboplatin, and prednisolone). In the mEPOCH regimen, carboplatin was used as a substitute for the cyclophosphamide in order to avoid the drug efflux pump in ATLL cells due to the expression of P-glycoprotein, which leads to multidrug resistance. In these retrospective analyses, the overall response rate and complete response rate were 58% and 25%, respectively, with a median follow-up of 8.9 months; the median survival time was 9.8 months (95% confidence interval, 7.2–13.9 months). The median progression-free survival (PFS) was 4.2 months (95% confidence interval, 3.4–5.7 months). Patients who completed ≥4 cycles experienced significantly better overall survival and PFS compared with those who completed <4 cycles. Twenty-eight patients underwent allogeneic hematopoietic stem cell transplantation after mEPOCH and demonstrated significantly prolonged overall survival and PFS compared with those who did not. The authors concluded the mEPOCH was effective with tolerable adverse effects and prolonged overall survival mainly in patients that underwent allogeneic hematopoietic stem cell transplantation [33].
Regarding multiple drug regimens for the treatment of ATLL, there is the question of which combination would be the most effective and/or in which type of ATLL it should be used. A retrospective analysis of transplant-eligible patients with ATLL who received only VCAP-AMP-VECP or CHOP, analyzed by propensity scoring of inverse probability of treatment weighting (IPTW), investigated which regimen is a preferable front-line therapy in patients with aggressive ATLL in intermediate- and high-risk groups. The results showed that from 947 and 513 patients treated with VCAP-AMP-VECP and CHOP, respectively, the crude probabilities of two-year overall survival (OS) for patients in the VCAP-AMP-VECP and CHOP groups were 31.2% and 24.6%, respectively (p < 0.001). Stratified by risk group according to the modified ATL-prognostic index score at diagnosis, the crude probabilities of two-year OS in the VCAP-AMP-VECP and CHOP groups were, respectively, 39.8 and 45.0% in the low-risk group (p = 0.69), 32.2 and 21.6% in the intermediate-risk group (p = 0.001), and 17.2 and 6.2% in the high-risk group (p = 0.005). The authors suggested that the VCAP-AMP-VECP regimen is a preferable front-line therapy in patients with aggressive ATLL in intermediate- and high-risk groups, but it is important to emphasize that the trial was rather small, and no subsequent studies confirmed the benefit of VCAP-AMP-VECP over CHOP [34].
An Important target explored for the treatment of ATLL is the C-C chemokine receptor 4 (CCR4). Almost all patients (≥90%) with ATLL over-express this receptor and the mogamulizumab, a humanized defucosylated anti-CCR4 monoclonal antibody, has been studied for this purpose, being legally approved in Japan for the treatment of CCR4-positive ATLL [35]. In a prospective, randomized therapeutic trial that evaluated the efficacy and safety of mogamulizumab in ATLL patients with acute, lymphoma, and chronic subtypes with relapsed/refractory, aggressive disease in the US, Europe, and Latin America, the investigators did not find a result in terms of tumor response; however, mogamulizumab treatment resulted in an 11% confirmed overall response rate, with a tolerable safety profile [36]. The overall response found was compared with a previous phase II study of mogamulizumab monotherapy in 26 Japanese patients with relapsed CCR4+ ATL, which showed a 50% overall response rate (ORR), but the authors affirm several differences may account for this discrepancy. For example, the Japanese study included relapsed, not refractory patients, and confirmation of response was evaluated after 4 weeks (compared to 8 weeks in the researcher’s research) [37].
However, the superiority of mogamulizumab over other salvage chemotherapies was not observed in general, with respect to overall survival (OS). It was only observed in patients with acute-type ATLL and skin rash. However, the assessment of skin adverse effects in clinical practice may be controversial, due to the subjective visual inspection of lesions [38].
The safety and effectiveness of mogamulizumab for the treatment of patients with (r/r) ATLL was also observed in another prospective, observational, post-marketing surveillance study with 572 patients, which concluded the best overall response rate and the response rate at the end of therapy were 57.9% and 42.0%, respectively. The median overall survival was 5.5 months, and survival was not different between patients aged ≥70 and <70 years, confirming it as a feasible option for the treatment of patients with r/r ATLL [39].
Drugs are also cited in the literature as useful for relieving symptoms and improving the quality of life of patients with ATLL. This is the case of etretinate, a synthetic retinoid analogue, which relieved the skin symptoms in cutaneous-type adult T-cell leukemia-lymphoma (cATL) at a daily dose of 10–40 mg in 9 cATL patients. The response of cutaneous lesions was evaluated by the modified response criteria for ATLL severity weighted assessment tools (mSWAT), before and 3 months after the introduction of the treatment. The remission was obtained in eight patients. Despite the adverse events observed in all patients but successfully controlled by the application of a moisturizer or a topical steroid, the limitations of the lack of controls, and the biases owing to the compliance or case selection, the authors suggest oral retinoids are a safe option for cATL patients, but studies with a larger sample population are needed [40].
Although clinical trials are extremely important to validate treatments, in vitro studies also play an essential role in the development of treatments. They help elucidate mechanisms of action and contribute to the search for new drugs or even the improvement of existing ones.
Among the drugs authorized for use by the US Food and Drug Administration and the European Medicines Agency, dimethyl fumarate (DMF) has been highlighted as a promising treatment for ATLL due to its effects in cancer cells, including cell signaling, proliferation, and cell death [41]. A study that examined the effects of DMF using the trypan blue exclusion assay and annexin V/propidium iodide staining in HTLV-1-infected and transformed T-cell lines (MT-1 and MT-2 cells), as well as evaluating its effects on the nuclear factor-kappa B (NF-ĸB), signal transducers, activators of transcription 3 (STAT3) signaling pathways, and anti-apoptotic proteins by immunoblotting, demonstrated the DMF inhibited proliferation and induced apoptosis in HTLV-1-infected and -transformed T-cells by suppressing NF-ĸB and STAT3 signaling pathways [42].
These results were observed by Maeta et al., and Sato et al., who evaluated the specific mechanism of DMF effects on the caspase recruitment domain family member 11 (CARD11)–BCL10 immune signaling adaptor (BCL10)–mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) (CBM) complex and upstream signaling molecules that are critical for NF-ΚB signaling in MT-2 cells by immunoblotting. In the cited study, DMF inhibited proliferation and influenced the apoptosis of HTLV-1-infected cells by suppressing the CBM complex in the NF-ΚB pathway [43][44].
In the literature, apoptosis is highlighted as one of the main routes by which drugs act in the treatment of ATLL. In this context, we have dorsomorphin, an AMPK inhibitor that induced apoptosis in PBMCs from ATLL patients and in HTLV-1-infected T-cell lines in a dose- and time-dependent form. It increased the production of intracellular reactive oxygen species (ROS) that mediated DNA damage in HTLV-1-infected T-cell lines and suppressed the growth of human ATLL tumor xenografts in NOD/SCID mice [41].
Another important strategy to treat ATLL would be the use of antibody drug conjugates (ADCs) that have been recently introduced as part of anticancer therapy [45]. Yokota et al. constructed a novel ADC composed by an anti-CD70 single-chain Fv-Fc antibody conjugated with emtansine and evaluated it with regards to cell cytotoxicity and target specificity assessed by a cell proliferation assay. The results showed the anti-CD70 ADC selectively killed HTLV-1-infected cells and ATL cells without affecting other cells; however, additional studies with a larger number of peripheral blood mononuclear cells (PBMCs) from different patients are warranted for establishing this potential [45].
Compounds from plants also have been studied for the treatment of ATLL. Hypericin (HY), a polycyclic quinone from Hypericum perforatum L., in photodynamic therapy (PDT), was tested against ATL cell lines. It was highly effective in inducing the inhibition of cell proliferation in an ATL cell with minimal effect on peripheral blood CD4+ T lymphocytes and caused apoptosis and G2/M phase cell cycle arrest in leukemic cells. Western blot analyses also revealed the downregulation of Bcl-2 and enhanced expression of Bad, cytochrome C, and AIF. The luciferase assay showed an enhanced expression of Bax and p53 proteins. Finally, the treatment suppressed the expression of viral protein HBZ and Tax by blocking the promoter activity via HTLV-1 5′LTR and 3′LTR, which highlighted the promising use of hypericin-PDT as therapy for ATLL [46].
Thymoquinone (TQ), obtained from Nigella sativa black seed, also was studied in combination with the anthracyclin doxorubicin (dox), whose major limitations in chemotherapy include tumor resistance and drug-induced severe side effects. TQ and dox caused greater inhibition of cell viability, induced apoptosis by increasing ROS and causing disruption of mitochondrial membrane potential, and reduced tumor volume in NOD/SCID mice more significantly than single treatments. Even though most of the mechanisms implicated in response to this combination model seem to be like those observed upon treatment with the drug alone, the authors suggest that this combination offers the possibility to use up to twofold lower doses of dox against ATLL with the same cancer inhibitory effects both in vitro and in vivo [47].
Below is an illustrative figure describing the pathological process of leukocyte mutation and the consequent damage leading to the development of ATLL, as well as the potential sites of action of the studied molecules. Figure 1 shows that the expression of Tax and HBZ in HTLV-1-infected cells results in continuous proliferation and inhibition of apoptosis, leading to cell immortalization. ATLL is associated with immunosuppression due to the accumulation of genetic alterations in infected T cells. Interactions with (E)-3-phenyl-5-(phenylamino)-2-styryl-1,3,4-thiadiazol-3-i lead to necrosis of infected cell lineages. The Fv-Fc anti-CD70 antibody induces the selective death of peripheral blood mononuclear cells in ATLL patients. Various therapies, such as dimethyl fumarate, dorsomorphin, etretinate, arsenic/interferon-alpha (As/IFN-α) with thymoquinone (TQ) [48], arsenic trioxide (As2O3) [49], and mogamulizumab, have specific actions to inhibit viral replication and induce apoptosis in HTLV-1-infected cells. Therapeutic plans VCAP-AMP-VECP or CHOP combine strategies to combat viral replication and immunosuppression. The modified EPOCH regimen and thymoquinone (TQ) with doxorubicin disrupt viral replication and induce apoptosis through free radical damage and the disruption of mitochondrial membrane potential in cells, respectively [50].
Figure 1. The pathological process of leukocyte mutation leading to the development of ATLL and potential sites of action of the studied molecules.

4. Drugs to Treat HTLV-1-Associated Myelopathy (HAM)/Tropical Spastic Paraparesis (TSP)

HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a progressive neuro-inflammatory disease [51]. HAM/TSP is characterized by spinal cord atrophy in the lower thoracic cord, perivascular demyelination, axonal degeneration, and inflammatory response caused by HTLV-1-infected CD4+ and CD8+ T cells [52]. This results in the destruction of nerve fibers, leading to a gradual loss of sensory-motor function. Symptoms include progressive muscle weakness in the lower limbs, stiffness, hyperreflexia, low back pain, paresthesia, and urinary and sexual problems [51][52][53].
Drug therapy for HAM/TSP caused by HTLV-1 cannot cure the disease. However, symptoms can be managed, and physiotherapy can contribute to patients’ functionality and quality of life, although the course of the disease is not altered. Currently, the use of corticosteroids, such as methylprednisolone in high doses and oral prednisolone (5 mg) for maintenance, is considered the best therapeutic option based on international consensus assessment and recommendations in the management of HAM/TSP to provide an evidence-based approach to the use of therapies made in accordance with the GRADE guidelines, a system for strength of evidence. According to this consensus, there is insufficient evidence to recommend interferon-alpha (IFN-α) as a first-line drug and antiretroviral therapy (treatment targeting HTLV-1 enzymes) for the treatment of HAM/TSP. It is important to emphasize that therapies to change the course of HAM/TSP must be applied to selected patients in order to maximize the benefits of treatment [19].
Although recommended, corticosteroid therapy has not yet been tested in a randomized clinical trial. Using this methodology, a phase 2 trial was performed to evaluate the effectiveness of corticosteroid therapy in patients with HAM/TSP. Patients were divided into two groups based on the speed of progression of motor impairments. In the fast-progressing group, those who received intravenous methylprednisolone plus oral prednisolone had one or more grade increase in the Osame motor impairment score, and 30% or more in the 10 m walk test from week 2 onwards. In the slow progression group, patients who received a single oral prednisolone treatment also showed a 15% improvement in the 10 m walk test, but only at the 24th week of the study [54].
In the last 5 years, some other drugs were studied as possible therapeutic options for HAM/TSP. Raltegravir, an antiretroviral that inhibits integrase, has been shown to successfully prevent in vitro transmission of HTLV-1 by interrupting the cell-to-cell contact necessary for the infection of healthy cells. Based on this result, a pilot, single-center, single-arm, open-label clinical study evaluated the effect of raltegravir on the proviral load and immune response of 16 patients with HAM/TSP. The patients received the antiretroviral drug twice a day for 6 months and were followed up for another 9 months without the drug. The results showed raltegravir did not equally reduce the proviral load in all patients. However, in the group in which there was a reduction in load, the expression of Tax and HBZ mRNA also decreased, with a reduction in Tax observed in the sixth month that was maintained until the fifteenth month of follow-up [55].
Teriflunomide, a dihydrooratate dehydrogenase inhibitor, was tested on samples from 12 patients with HAM/TSP caused by HTLV-1. Patients were not using corticosteroids and had been diagnosed with HTLV-1 for over 10 years. The collected cells were cultured and subjected to tests to assess lymphocyte proliferation; expression of activation markers, Tax, and HBZ mRNA; and Tax protein expression. The results showed that teriflunomide exerted a dose-dependent regulatory action on the spontaneous proliferation of lymphocytes. The best response was observed at the 100 μM dose, resulting in a 90.7% inhibition, preventing the abnormal proliferation of CD4+ and CD8+ T lymphocytes, although markers of lymphocyte activation, such as CD25, were still expressed. There was no change in Tax and HBZ mRNA expression, as well as in Tax protein expression [56].
HAM/TSP also causes urinary disorders, such as an overactive bladder, and affects patients’ quality of life. In the search for alternatives to treat these symptoms, a prospective, single-center, open-label study evaluated the 12 week use of 300 mg of prosultiamine, a homologue of allithiamine, in 16 patients with HAM/TSP-related overactive bladder, evaluating nocturnal urination frequency, urinary urgency, and nerve growth factor/creatinine and adenosine. There was a decrease in biomarker concentrations and less nighttime urination and urinary urgency, with no reports of relevant adverse effects during the trial [57].
Below is an illustrative figure depicting the pathological process of leukocyte infiltration and the consequent injury that leads to the development of clinical manifestations associated with HAM/TSP, as well as the possible sites of action of the studied molecules. The HAM/TSP is related to the production of IFN-γ (interferon-gamma) and NFG (nerve growth factor) by HTLV-1-infected cells and HTLV-1-specific cytotoxic cells infiltrating the cerebrospinal fluid, leading to damage to neural tissues, both directly through the exacerbation of inflammatory responses and indirectly by recruiting more IFN-γ-producing cells into the cerebrospinal fluid. Thus, the action of antiretrovirals raltegravir [55] and teriflunomide [56] is focused on reducing the expression of the proteins Tax and HBZ [49], which are essential for viral replication and regulating the expression of new CD4+ and CD8+ T cells. On the other hand, the intravenous methylprednisolone plus prednisolone oral treatment showed a reduction in the rate of pro-inflammatory cytokines produced by astrocytes [54]. Finally, prosultiamine demonstrated improvement in the inflammatory response by reducing the expression of IFN-γ and NFG [57], a mechanism different from the one proposed by L-arginine supplementation, which possibly shows a reduction in leukocyte infiltration through the reduction in neopterin levels [58] (Figure 2).
Figure 2. Pathological process associated with HAM/TSP and possible sites of action of the studied molecules. Adapted from Futsch, Mahieux, Dutartre et al., 2017 [50].

5. Molecular Docking as a Tool to Discover New Drugs against HTLV-1 Infection and Related Diseases

A method prominent in recent studies for drug discovery is molecular docking (MD). This technique is used to predict the preferred position of a binding molecule in a receptor macromolecule and evaluate physical-chemical properties related to their biological activity [59]. MD can identify new drugs for the treatment of HTLV-1 through the prediction of interactions between small molecules and essential proteins in the targeting of viral enzymes and host factors involved in both HTLV-1 replication and problems associated with infection [60].
Studies have described the progression of HTLV-1 infection, inflammatory conditions, and the occurrence and severity of related diseases as consequences of HTLV-1 replication in CD4+ T and CD8+ T lymphocytes, as well as the imbalance between proinflammatory and anti-inflammatory cytokines [61][62]. There are several protein targets in HTLV-1, such as protease (PR), reverse transcriptase (RT), and integrase (IN), which are ultimately involved in viral replication and retroviral pathogenesis [63]. For example, HTLV-1 protease is an aspartic protease crucial for the maturation and replication cycle of HTLV-1, with 28% similarity to HIV-1 protease and 45% identity between active site residues. The blockage of HIV-1 PR enzyme has shown to be reliable in battling the virus [63]. Additionally, HTLV-1 PR is important for viral growth and replication by cleaving the viral Gag and Gag-(Pro)-Pol polyproteins and aiding the maturation of structural and functional proteins of the virus [64]. Consequently, hindering HTLV-1 PR can be a reliable treatment against HTLV-1 infections, demonstrating that some target proteins of anti-HIV treatment may be promising for anti-HTLV-1 treatment [65].
In addition, the cost of drug discovery and development can reach up to 2.6 billion dollars [66], but by using drug repurposing, the cost can be drastically reduced because it is a process of finding new indications for already tested and approved drugs, which can be very efficient and timesaving, as all the safety tests have been done and only the efficacy of the potential drug must be tested in clinical trials. Even the design of new antiretrovirals more specifically focused on HTLV-1 targets based on existing antiretrovirals can save money and time. Computational tools and methods such as molecular docking and molecular dynamics (MD) simulation are widely used methods in the field of drug discovery and drug design because they enable the study of 3D structures and interactions of protein–ligand complexes in great detail with the possibility of finding potential binders [60].
The recent study by Jahantigh et al. (2022) used molecular docking to identify potential protease inhibitors among approved antiviral drugs as a potential treatment against HTLV-1. In general, protease is an essential enzyme for the viral replication cycle and therefore represents a promising target for the development of antiviral therapies. In the study, molecular docking techniques and molecular dynamics simulation were used to evaluate the interaction of different antiviral drugs with HTLV-1 protease [64]. The results showed that some antiviral drugs (mainly simeprevir, atazanavir, and saquinavir), originally developed to fight other viruses, presented a significant affinity with the HTLV-1 protease. This finding suggests the possibility of repurposing these drugs for the treatment of HTLV-1 infection that could speed up the process by leveraging already available knowledge and resources [64].
To take advantage of knowledge about existing antivirals and reconstruct the binding pathway of an anti-HIV drug indinavir, in a complex with HTLV-1 protease, Sohraby and Hassan (2021) used molecular dynamics simulations. Indinavir is a protease inhibitor, and the simulation made it possible to follow the molecular pathway of indinavir binding involved in this process. The reconstruction of the binding pathway of indinavir with the HTLV-1 protease provided valuable information on the molecular mechanisms involved in this interaction for developing new treatment solutions against HTLV-1 [67].

This entry is adapted from the peer-reviewed paper 10.3390/ph16111546

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