Potential therapeutics

A recent high throughput study identified that the FDA approved

immunosuppressive drug FK506 (tacrolimus) was able to increase expression of BMPR2 (Spiekerkoetter et al., 2013). This suggested that FK506 had the potential

to be used as a therapy for PAH as the reduced BMPR2 expression in PAH

patients, even those without a loss of function mutation, is thought to be a key driver in the development of disease. FK506 induced BMPR2 signalling through its action as a calcineurin inhibitor and through its binding to FK-binding protein 12 which is a repressor of bone morphogenetic protein (BMP) signalling. The

mechanism of action proposed by Spiekerkoetter et al (2013) is shown below in Figure 1.4. Treatment with FK506 in the monocrotaline rat model and the Sugen hypoxia (SuHx) rat model showed improvements in all parameters of PH

(Spiekerkoetter et al., 2013). FK506 significantly reduced RVSP, RVH (as measured by RV/LV + Septum) and pulmonary artery remodelling in all models investigated. However, this study was performed in male mice and rats only and therefore was lacking any investigation into the sex-dependent effects of this treatment.

Figure 1.4 - Proposed mechanism of action for the activation of BMPR2 signalling by FK506.

A-B) Ligand is bound to BMPR2 dimerized with Type 1 Receptor. A) A sub-activating ligand binding does not lead to BMPR2 signalling. B) Activating BMP ligand binds leading to FKBP12 and Calcineurin dissociating from receptor and normal BMPR2 signalling. C-D) Ligand binds to mutant BMPR2 dimerized with Type 1 Receptor. C) Activating BMP ligand binds resulting in abberant BMPR2 signalling. D) Sub-activating ligand binds in the presence of FK506. FK506 acts to inhibit calcineurin and binds FKBP12. This releases inhibitory effect on BMPR2 signalling even in the presence of BMPR2 mutation. BMP – bone morphogenetic protein. BMPR2 – bone morphogenetic protein receptor 2.

FKBP12 – FK binding protein 12.

Sub-activating ligand

The same group also went on to trial compassionate use of low dose FK506 in patients with late stage, severe PAH for whom there are no other treatment options available (Spiekerkoetter et al., 2015). Three female patients aged 36, 50 and 55 were treated with FK506 and monitored throughout the treatment regime. At 12 months, 2 of the 3 patients showed improved or stable RV function in relation to ejection fraction, stroke volume index and cardiac output index.

Neither of the patients had a PAH related hospitalisation within the 12-month period. The third patient unfortunately had to discontinue FK506 treatment as a result of significant clinical worsening, however, once stabilised the patient voluntarily started on FK506 once again and at 12 months of round 2 of FK506 also hadn’t had any PAH related hospitalisations. This study showed promise that FK506 could significantly improve the quality of life and survival rates of PAH patients.

A Phase IIa clinical trial conducted by the same group in the USA was published in 2017 (Spiekerkoetter et al., 2017). This trial aimed to determine the safety and tolerability of low dose-FK506 in PAH patients. Although the study showed that FK506 was reasonably well tolerated in all patients with nausea/diarrhoea being reported as the most common side effect, expression of BMPR2 and parameters of PAH were not significantly improved across the patient cohort.

There was a trend towards improvement in 6 min walk distance and in

measurements associated with heart failure however only in some patients and the changes did not reach significance. It was suggested that a phase IIb efficacy trial would be appropriate but, currently, there is not a trial underway.

1.5.2.2 Aromatase inhibitors

Aromatase inhibitors act to prevent the production of endogenous estrogen from the androgens - androstenedione and testosterone (Kelly and Buzdar, 2010;

Grodin et al., 1973). As studies have indicated a pathogenic effect of estrogen in the pulmonary vasculature, aromatase inhibitor – Anastrozole – is being

considered as a therapeutic for the treatment of PAH. Anastrozole is currently licenced for the treatment of hormone receptor positive breast cancer (Geisler et al., 1996; Baum et al., 2002). It reversibly binds to the haem group of

aromatase inhibiting its catalytic activity (Kelly and Buzdar, 2010). The half-life of the drug is approximately 50 hours and within 7- 10 days a steady level within

the plasma can be achieved (Kelly and Buzdar, 2010). As this therapy is already licenced for the treatment of breast cancer and its efficacy is well studied and it is already known to have limited side effects this makes it an attractive

therapeutic for PAH patients.

Initial pre-clinical studies showed that treatment with Anastrozole at 3 mg/kg/day significantly reversed PH disease phenotype in female chronic hypoxia mice and female Sugen hypoxia rats (Mair et al., 2014b). This finding was not replicated in male mice. Anastrozole treatment in female BMPR2 mutant mice showed its ability to improve parameters of metabolic dysfunction (Chen et al., 2017). The demographics of PAH patients has changed in recent years with more patients presenting with other co-morbidities such as obesity. These patients will have significantly more adipose tissue which is a well-established site of estrogen production. Treatment with anastrozole significantly improved RVSP in ob/ob male and female mice that spontaneously develop PH (Mair et al., 2019). Almost a third of PAH patients are clinically obese when they present at diagnosis, therefore these findings are relevant to the treatment of the current PAH population (Farber et al., 2015).

These findings in pre-clinical models led to a small Phase II trial in male and female PAH patients to assess the safety and tolerability (Kawut et al., 2017).

Anastrozole successfully reduced circulating 17β-estradiol levels by

approximately 40% after 3 months of treatment. The study indicated that Anastrozole was well tolerated by PAH patients and was capable of improving 6 min walk distance in these patients. The effect of Anastrozole on other disease parameters assessed such as functional class or adverse events was not as clear.

One potential limitation of the use of Anastrozole to treat PAH patients is that estrogen has been shown to be cardioprotective (Iorga et al., 2017) and although treatment may improve lung pathology it may have a detrimental effect on the RV function, the main determinant in PAH patient survival. This small Phase II did not highlight any adverse effects of Anastrozole treatment on RV function predominantly assessed by tricuspid annular plane systolic excursion (TAPSE) measurements. This study indicated a larger scale clinical trial was appropriate.

The PHANTOM Phase II clinical trial is currently recruiting patients

(NCT03229499). This clinical trial will investigate the effect of Anastrozole

versus placebo treatment on 6 min walk distance and other clinical parameters of PAH during a 6-month period. The safety and tolerability of the drug will also be investigated over a 12-month dosing period.