# Medical use of arsenic trioxide

Arsenic trioxide :
| ATC code | L01XX27 |
| use during pregnancy | category D |
| half-life | 92 hours |
| plasma protein binding | insignificant |
| metabolism | methylation |
| excretion | through urine (60% within 8 days) |
| routes of administration | intravenous |
Medical use of arsenic trioxide refers to the use of arsenic trioxide (Latin: Arsenum trioxydatum, also known as "arsenic") as an chemotherapeutic agent in the treatment of acute promyelocytic leukemia. Arsenic trioxide has orphan drug status and is available as the pharmaceutical preparation Trisenox. When in contact with water, it forms arsenous acid, which is believed to be the biologically active substance.
The action of this substance involves inhibiting the proliferation of cancer cells and inducing their differentiation or apoptosis, although the exact mechanism of the drug's action is not fully understood. Arsenic, due to its toxic properties, had been used for centuries as an effective and virtually undetectable poison for the senses. In the 20th century, its anticancer properties were observed, but attempts at oral administration were unsuccessful. Only intravenous administration of the substance yielded positive results, particularly in the treatment of a rare form of cancer – acute promyelocytic leukemia. This therapy is introduced after failure of treatment with retinoids and chemotherapy. The treatment is characterized by relative safety and few side effects. Research is ongoing to find other uses for the drug.

## History

In the 18th century, William Withering discovered that arsenic trioxide, when used in small doses, exhibited therapeutic effects. During the same period, Thomas Fowler prepared a 1% solution of arsenic and potassium carbonate, which was used to treat skin diseases (primarily psoriasis) until the 20th century. An arsenic-based drug, arsphenamine, was also developed for the treatment of syphilis, synthesized by Paul Ehrlich, though it was eventually replaced by penicillin. Arsenic compounds were widely used to treat various diseases in the 19th and early 20th centuries.
The first reports of the anticancer activity of arsenic trioxide date back to 1878, when a report from Boston City Hospital described Fowler's solution lowering leukocyte levels in the blood of two healthy individuals and one patient. Arsenic trioxide continued to be used in the treatment of leukemia until the introduction of radiotherapy. It made a resurgence in the 1930s when the first studies confirmed the high efficacy of arsenic trioxide in treating chronic myelogenous leukemia.
In the late 1960s, physicians working at the Harbin Medical Academy in China were sent to a center focusing on traditional Chinese medicine, where they used a melanoma ointment, with arsenic as its main ingredient. At that time, the arsenal of anticancer drugs was limited, prompting doctors to experiment with arsenic. Early trials used oral administration, but it showed strong toxic effects. In March 1971, the first trials of intravenous arsenic began, which showed significantly lower toxicity. For many years, arsenic trioxide was administered to patients with various cancers, showing the best results in the treatment of acute promyelocytic leukemia. More than half of the patients from the first trial in Harbin survived for five years, prompting further research across other centers in China, and eventually at the Sloan-Kettering Memorial Institute in New York. The results of clinical trials were favorable enough that in 2000, the drug received FDA approval.

## Mechanism of action

The mechanism of action of arsenic trioxide is complex and not fully understood. Generally, the drug inhibits the proliferation of cancer cells and induces their differentiation and/or apoptosis, which can occur in various ways depending on the involved organelles and biochemical processes. Arsenic trioxide induces apoptosis through:
* Interaction with cell membrane receptors (extrinsic pathway)
* Interaction with mitochondria (intrinsic pathway)
The first of these pathways involves the binding of a ligand to a receptor located on the surface of the cell membrane. The interaction of these two entities leads to the activation of various genes and releases a cascade of proteins characteristic of the apoptosis process.
Arsenic trioxide also interacts with mitochondria. One of the initial changes in their structure induced by the drug is the opening of megachannels and the release of so-called "death proteins", primarily cytochrome c, APAF-1 (apoptotic peptidase activating factor 1), AIF (apoptosis-inducing factor), Smac/DIABLO protein, and endonucleases from the intermembrane space of mitochondria into the cytosol. In the cytoplasm, a protein complex known as the apoptosome forms, which activates further processes leading to apoptosis.
Regardless of whether apoptosis is induced externally or internally, it always involves caspases, whose activation irreversibly leads the cell down the path of programmed cell death. Additionally, apoptosis is regulated by proteins from the Bcl-2 family, which can act as either pro-apoptotic or anti-apoptotic factors.
The cause of acute promyelocytic leukemia is the translocation of the gene encoding the retinoic acid receptor (RARα) from chromosome 17 to a location near the PML gene on chromosome 15. This leads to the fusion of genes and the production of the PML/RARα protein. This protein inhibits differentiation and the death of the cells in which it is present. Arsenic trioxide, even at low concentrations, causes the degradation of PML/RARα, thereby partially restoring the differentiation of cancerous promyelocytes.
Arsenic trioxide activates JNK (c-Jun N-terminal kinase), also known as stress-activated protein kinase, which belongs to the MAPK (mitogen-activated protein kinase) family. These enzymes play a crucial role in signal transduction within the cell. Under normal conditions, JNK is activated by the phosphorylation of threonine and tyrosine residues. However, studies on specific cell lines derived from patients with acute promyelocytic leukemia have demonstrated that this activation also occurs in response to arsenic trioxide.
It seems that the activation of JNK leads to the phosphorylation of both anti-apoptotic proteins (Bcl-2, Bcl-Xl) and pro-apoptotic proteins – Bax (Bcl-2-associated X protein), Bak (Bcl-2 homologous killer), and Bid (BH3 interacting domain death agonist) – effectively activating them. Pro-apoptotic proteins contain the BH3 domain, which is responsible for their "death-inducing" activity. They cause the formation of ion channels in the mitochondrial membrane, resulting in the release of the aforementioned apoptotic factors into the cytoplasm. Anti-apoptotic proteins owe their function to a hydrophobic cleft in their spatial structure that binds to the BH3 domain, thereby neutralizing the effects of the "death" proteins. Under normal conditions, the decision for a cell to undergo apoptosis depends on the ratio of pro-apoptotic to anti-apoptotic proteins. In the case of arsenic trioxide-induced apoptosis, two mechanisms play a significant role in increasing the levels of pro-apoptotic proteins. The first is related to the functioning of the transcription factor NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells). NF-κB exists in the cytoplasm in an inactive state, in a complex with the specific reaction inhibitor IκB (IKK). This complex consists of two catalytic subunits – IKKα and IKKβ – and a regulatory unit IKKγ/NEMO. The phosphorylation and degradation of the inhibitor release NF-κB, which then translocates to the cell nucleus and activates genes responsible for producing "survival" proteins (such as p53, Bcl-2, and other inhibitors of apoptosis). NF-κB also protects cells from apoptotic stimulation involving the TNF-α receptor. Arsenic trioxide binds to the cysteine at position 179 of IKKβ, thus preventing the release of NF-κB. The absence of this protein in the cytoplasm allows for the induction of apoptosis via the extrinsic pathway and activates caspases 3 and 8.
This mechanism has been observed not only in acute promyelocytic leukemia cells and Hodgkin lymphoma but also in patients with myelodysplastic syndrome. The second mechanism that increases the levels of pro-apoptotic proteins is the downregulation of bcl-2 gene transcription. This effect has been observed in HL-60 and NB4 human leukemia cells.
In 2003, Japanese researchers discovered that arsenic trioxide induces apoptosis not only through the TNF-α receptor. Studies indicate that the drug also acts pro-apoptotically through the CD95 receptor, which affects the activation of caspases 8 and 3. In multiple myeloma cells, arsenic trioxide interacts with the APO2/TRAIL receptor, activating caspases 8 and 9.
Arsenic trioxide also affects the intracellular concentration of glutathione, which is a crucial component of the redox system (it removes radicals and reduces hydrogen peroxide). It also participates, along with peroxidase and catalase, in regulating the levels of reactive oxygen species. Arsenic trioxide inhibits glutathione peroxidase, thereby decreasing its concentration in the cell, which leads to an increase in the levels of reactive oxygen species. These, in turn, increase the permeability of the mitochondrial membrane, causing the release of apoptotic factors and initiating the apoptosis process.
Additionally, arsenic trioxide degrades poly(ADP-ribose) polymerase, which, combined with the activation of caspases, inhibits DNA repair and halts the cell cycle. The phase of the cell cycle at which the blockage occurs primarily depends on the p53 protein. In cells containing the so-called "wild type" (non-mutated) p53, the cell cycle is halted in the interphase, while in cells with mutated p53, it is halted in the G$_2$/M phase.

## Clinical studies

Arsenic trioxide was clinically tested in two open-label, single-arm trials without a control group, involving 52 patients with acute promyelocytic leukemia who had previously been unsuccessfully treated with anthracyclines and retinoids. The results of these studies are presented in the table below.
| Study | Single-center n=12 | Multi-center n=40 |
| Dose of arsenic trioxide (mg/kg body weight/day) | 0.16 (median: 0.06–0.20) | 0.15 |
| Complete remission | 11 patients (92%) | 34 patients (85%) |
| Average time to bone marrow remission | 32 days | 35 days |
| Average time to achieve complete remission | 54 days | 59 days |
| 18-month survival rate | 67% | 66% |
| n – number of patients participating in the study |||
Studies have also been conducted on the effect of arsenic trioxide on other cancers. These showed that the drug also induces apoptosis in lung cancer cells (especially in combination with sulindac). The efficacy of arsenic trioxide has also been demonstrated in the treatment of multiple myeloma, in combination with ascorbic acid and bortezomib.
Animal studies have shown that the drug also affects ovarian, liver, stomach, prostate, and breast cancers, as well as gliomas and pancreatic cancer (in combination with parthenolide). However, attempts to use arsenic trioxide in the treatment of solid tumors have been limited by the drug's toxicity.
Arsenic trioxide also appears promising for treating autoimmune diseases (based on studies in mice).

## Pharmacokinetics

Detailed pharmacokinetic studies on arsenic trioxide have not been conducted. When administered intravenously, a steady state is reached after 8–10 days. Arsenic binds to proteins to an insignificant extent. The highest concentrations of arsenic are found in the liver, kidneys, heart, lungs, hair, and nails. Arsenous acid is oxidized to arsenic acid and methylated in the liver, and then excreted 60% in the urine. The drug has a half-life of 92 hours. Arsenic trioxide is neither a substrate nor an inhibitor of cytochrome P450 isozymes (1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4/5, 4A9/11).

## Indications

Arsenic trioxide is intended for the induction of remission and consolidation in adult patients with acute promyelocytic leukemia who have the t(15;17) translocation and/or the PML/RARα gene. The drug should be used after treatment failure or relapse. Prior therapy should include retinoid and chemotherapy.

## Special warnings

To ensure the safe use of arsenic trioxide, the following precautions should be observed:
* 25% of patients treated with arsenic trioxide exhibited symptoms resembling leukocyte activation syndrome, characterized by high fever, shortness of breath, weight gain, pulmonary infiltrates with pleural or pericardial exudation, with or without leukocytosis. High doses of steroids (10 mg dexamethasone intravenously, 2–3 times per day) appear to alleviate these symptoms.
* 40% of patients treated with arsenic trioxide experienced at least one instance of prolonged QT interval, corrected to over 500 ms. QT interval prolongation can lead to ventricular arrhythmias, such as torsades de pointes.
* Prior to initiating arsenic trioxide treatment, an ECG should be performed, and blood levels of potassium, calcium, magnesium, and creatinine should be checked. Any abnormalities, particularly a prolonged QT interval on the ECG, should be corrected before starting arsenic trioxide. Any medications that may prolong the QT interval should be discontinued if possible.
* Patients receiving arsenic trioxide, particularly those at risk for torsades de pointes, should be closely monitored during treatment.
* If toxicity reaches level 3 (as per National Cancer Institute criteria), treatment should be modified or discontinued before the planned completion of therapy. Patients can resume treatment only after symptoms subside, starting with 50% of the prior daily dose. The dose can be increased to the previous level if no toxicity symptoms appear within 3 days. If toxicity reappears, treatment with arsenic trioxide cannot continue.
* During the induction phase, electrolyte levels, glucose, blood counts, and liver and kidney function should be tested twice a week. In the consolidation phase, these tests should be performed weekly.
* Caution is advised in patients with kidney failure.
* During arsenic trioxide treatment, women of childbearing age and men capable of fathering children should use effective contraception. The impact of arsenic trioxide on fertility has not been thoroughly studied.

## Interactions

Arsenic trioxide is known to prolong the QT interval. If possible, medications that also prolong QT should not be used concurrently, including:
* Class Ia and III antiarrhythmics (e.g., quinidine, amiodarone, sotalol, dofetilide)
* Certain psychiatric medications (e.g., amitriptyline, thioridazine)
* Some macrolides (e.g., erythromycin)
* Some antihistamines (e.g., terfenadine, astemizole)
* Some quinolones (e.g., sparfloxacin)
* Other QT-prolonging drugs (e.g., cisapride)
Prior use of:
* Anthracyclines
* Amphotericin B
* Non-potassium-sparing diuretics
* Other drugs causing hypokalemia or hypocalcemia
increases the likelihood of torsades de pointes.

## Side effects

Side effects were reported in 37% of patients treated with arsenic trioxide. However, these effects were generally mild and resolved during treatment. Patients tolerated consolidation therapy better than induction therapy. The most common side effects include:
* Hyperglycemia
* Hypokalemia
* Neutropenia
* Elevated AlAt levels
* Leukocytosis
Severe adverse effects are relatively rare and include:
* Leukocyte activation syndrome
* QT interval prolongation (only one case of torsades de pointes was noted during clinical trials)
* Atrial fibrillation or flutter
Other side effects include allergic skin reactions (including reactions at the injection site, injection site pain), gastrointestinal disturbances (diarrhea), various types of pain, visual disturbances, and bleeding. If the drug extravasates, local irritation and phlebitis may occur.

## Overdose

In the event of arsenic poisoning (manifesting as seizures, muscle weakness, confusion), the administration of the drug should be immediately discontinued, and appropriate treatment should be initiated. Penicillamine is commonly used at a dose of up to 1 g/day. For patients unable to take oral medications, dimercaprol can be administered intramuscularly at a dose of 3 mg/kg body weight every 4 hours until life-threatening symptoms subside. In cases of coagulopathy, DMSA is recommended at a dose of 10 mg/kg body weight every 8 hours for 5 days, followed by every 12 hours for 2 weeks. Kidney dialysis may also be considered.

## Preparations and form of the drug

* Trisenox – Almac Pharma – 1 mg/ml concentrate for intravenous therapy. Trisenox is packaged in 10 ml ampoules for single use. The ampoules contain a pure solution of arsenic trioxide, without preservatives, and also contain sodium hydroxide and hydrochloric acid. The solution has a pH of 7–9. The drug should be stored at room temperature and must not be frozen. After withdrawing the solution from the ampoule, it should be diluted in 100–250 ml of 5% glucose or saline solution. Arsenic trioxide should not be mixed or administered in the same infusion with other medications.
* In pharmaceutical compounding, arsenic trioxide was used in a 1:10 trituration with lactose (Trituratio Acidi arsenicosi 1/10). To prepare the trituration, one part arsenic trioxide is placed in a mortar, and while continuously grinding, nine parts of lactose are added in portions. Achieving a uniformly distributed trituration requires at least 10 minutes of grinding in the mortar.