Histone Deacetylase Inhibitors as Therapeutics for Endometriosis

Xin Li; Xishi Liu; Sun-Wei Guo


Expert Rev of Obstet Gynecol. 2012;7(5):451-466. 

In This Article

Obstacles to Testing HDACIs

Despite all these published in vitro, in vivo and even preliminary clinical studies showing desirable features of HDACIs as a potential therapeutic agent, so far, to the best of the authors' knowledge, there has been no clinical trial that evaluates the efficacy of HDACIs on endometriosis. Given the current situation in clinical trials of endometriosis, this is somewhat surprising, especially because there are not many viable options when it comes to choosing compounds with preclinically demonstrated therapeutic potentials and known pharmacology, toxicity, pharmacokinetics and pharmacodynamics. Since VPA, as an inhibitor of class I and II HDACs, is the only HDACI that has been used, as a drug, widely and for decades, some possible reasons for not using it in the treatment of endometriosis are enumerated. It should be noted, however, that some other HDACIs have been approved for treating certain cancers. SAHA, for example, has recently received US FDA approval for the treatment of advanced cutaneous T-cell lymphoma, although its long-term side effects are not clear.

Potential Detrimental Effects of VPA: Myth or Reality?

Global hypomethylation is reported to be a notable feature of cancer that may cause genomic instability.[112,113] Therefore, there is a legitimate concern as to whether the therapeutic use of DMAs and/or HDACIs in treating endometriosis would increase the risk of cancer. Understandably, endometriosis is not a life-threatening disease even if left untreated. Thus, there is a question as to whether the benefit of HDACI therapy can outweigh or offset its risk.[50]

Published studies have indicated that a small percentage (0.2–3%) of silenced genes are upregulated by DMA treatment in cancer cells.[114,115] In normal fibroblast cell lines, the number of genes affected is still lower (0.4%.)[116] Similarly, HDACIs also upregulate a small subset of genes (0.4–0.8%) and downregulate an even smaller subset of genes in multiple myeloma cell lines.[115,117] In fact, the upregulated genes are involved in apoptosis, cell cycle, cell adhesion, proliferation and cell migration; in other words, in important cancer-related pathways.[115,117]

VPA was approved as an antiepileptic drug (AED) in 1967 in France and in 1978 in the USA, and has become the most widely prescribed AED worldwide. VPA has a fairly short elimination half-life: the mean terminal half-life for VPA monotherapy ranged from 9 to 16 h following oral dosing regimens of 250–1000 mg. VPA is listed in Pregnancy Category D by the US FDA, that is, there is clear evidence of risk of teratogenecity, but the benefits may outweigh the risk for pregnant women who have a serious condition that cannot be treated effectively otherwise. This is mainly due to its teratogenic risk associated with daily doses of 1000 mg or more and/or polytherapy.[118]

VPA has been shown to induce histone acetylation, DNA demethylation and expression of an ectopically methylated transiently transfected DNA in a dose-dependent manner in human embryonal kidney 293 cells.[119] A follow-up study also indicates that, by changing the extent of acetylation, VPA can alter the state of expression of genes, which are stably suppressed by DNA methylation,[120] including metalloproteinase 2, which is involved in endometriosis. Similarly, in nuclear extracts from adult mouse brain, VPA is found to induce DNA demethylation even though it does not inhibit the expression of DNMT1 and DNMT3A, the two most abundant forms of DNMTs in mammalian brains.[121] In support for the use of VPA to alleviate manic episodes in psychotic and depressed patients by enhancing GABAergic transmission, the same study also shows that treatment with VPA upregulates reelin (Reln) and glutamic acid decarboxylase 67 (Gad67) mRNA/protein expression by reducing the methylation of their promoters.[121] In HeLa (a cervical cancer cell line) cells, VPA treatment is found to decondense chromatins, but no changes in chromosome abnormalities, mitotic indices or morphologically identified cell death are found under VPA doses close to the therapeutic antiepileptic plasma range.[122]

Unfortunately, there seem to be no data, at least in the public domain, on whether expression levels of any gene are significantly changed after treatment with HDACIs, what genes these are and their percentage in normal endometrial cells. Regardless, although the percentage of affected genes is generally small, it remains a possibility, albeit remotely, that these affected genes may be important enough to cause unacceptable harm to the woman taking the drug or to her fetus. In addition, a rapid return of methylation has been reported following withdrawal of methyltransferase inhibitors (such as DMAs).[123] This may imply that it may require continued or long-term drug treatment in order to achieve a long-lasting epigenetic reprogramming.

However, even with these legitimate concerns, it should be noted that two DMAs – 5-azacytidine and 5-aza-2′-deoxycitidine (decitabine) – appear to be well tolerated and are not associated with significant demethylation of repetitive elements or any indication of secondary malignancies.[124] Notably, chromosomal abnormalities were even reported to be reversed in 31% of patients with myelodysplastic syndrome (MDS) who took decitabine.[125] The two drugs have therefore now been approved in the USA for the treatment of MDS and are the only agents known to improve the clinical outcome of MDS.[126]

Despite the concern for carcinogenesis because of hypomethylation, there are no credible data suggesting that VPA is carcinogenic. One animal study reported that VPA administered orally to rats and mice at doses of 80 and 170 mg/kg/day (less than the maximum recommended human dose on a mg/m2 basis) for 2 years was found to be associated with an increase in the incidence of subcutaneous fibrosarcomas in male rats receiving high-dose VPA and a dose-related trend for benign pulmonary adenomas in male mice.[127] However, a population-based case–control study reported that long-term use of VPA is not associated with an increase or decrease of cancer risks.[128] Currently, there are no data demonstrating that VPA is mutagenic in humans.[202]

Concern also has been raised that VPA use may increase the risk of polycystic ovarian syndrome.[129] However, in the same study that formed the basis of these concerns, the authors actually stated explicitly that "[N]one of the tested AEDs influenced 3βHSDII or P450c17 activities at concentrations normally used in AED therapy"[130] (italicized for emphasis). As Paracelsus said, only dose makes the poison. Accordingly, the extrapolation of observations made with high dosages to situations of low dosage should be undertaken with caution.

As with dose, the duration of treatment may also be important. In particular, the two studies that formed the basis of the concerns examined women taking VPA for ≥2 years.[131,132] Even if VPA is proven to have an unacceptable risk–benefit ratio, it may still be premature to conclude that all HDACs are useless, considering that VPA is just one of many HDACIs and some novel HDACIs may still have an acceptable risk–benefit ratio.

One way, thus far unexplored, to improve drug safety while minimizing potential side effects is to use the alternative drug delivery route, for example, the drug-containing intrauterine system (IUS). Indeed, it has been reported that levonorgestrel-releasing IUS appears to be efficacious in treating endometriosis, adenomyosis and their associated pain.[133] A recent mouse efficacy study of adenomyosis has shown that the danazol-loaded IUS is superior to the systemic administration of danazol in treating adenomyosis.[134] By changing the systemic to a more localized, perhaps more targeted, delivery route, it is possible to achieve therapeutic efficacy while minimizing untoward side effects. Unfortunately, little work has been done in this area.

Then, there is also an issue regarding whether VPA represents the best HDACI in treating endometriosis. It is unclear as to whether pan or specific HDACIs can have a better risk–benefit ratio than VPA. Regardless, the long-term safety of HDACIs and/or DMAs of therapeutic use should be carefully evaluated even though they prove to be efficacious in treating endometriosis. It is likely that such data may first come from clinical cancer trials.

One final point is that many natural products actually contain weak HDACI activity. For example, royal jelly is reported to have weak HDACI activity.[135] Except for allergic reactions, royal jelly can be and has been consumed safely by humans. This demonstrates that HDACI activity, at least in the weak form, does not necessarily cause detrimental health effects.

Lack of Financial Incentive

VPA was approved by the US FDA for treating epilepsy in 1978. The patent has long since expired. While drug companies can file patents on the use of VPA for new indications such as endometriosis and/or adenomyosis, there are already several generic VPAs on the market. Consequently, there is lack of strong financial incentive for conducting expensive clinical trials to evaluate the therapeutic potential of VPA in treating endometriosis.

The fact that VPA is a Pregnancy Category D drug also means that, for treating a disease that affects women of mostly reproductive age, there are certain inherent risks associated with the drug, which may be another deterrent to pharmaceutical companies to launch trials.

Yet this kind of thinking may be too myopic. In the case of a well-designed and well executed clinical trial testing, VPA would yield a great deal of information on the risks and benefits of the compound tested. In addition, for VPA, a wealth of pharmacokinetic, pharmacodynamic and toxicological data are available, and there is no need for Phase I trials. Some compounds may show fantastic preclinical results, yet fail in Phase I trials because of safety concerns. VPA will probably not have this issue. Its chemical makeup, stability, solubility and production scalability – many aspects of drug development involved with satisfying the regulatory requirements of drug licensing authorities before an Investigational New Drug can be approved – are known facts. With the known contraindications,[118] the efficacy and the risk–benefit ratio can be properly evaluated in clinical trials.