**1. Introduction: The Therapeutic Challenge**

Renal cancer ranks twelfth in incidence among cancers worldwide and has a lifetime risk of 1 to 63 for a given individual, with numbers estimated to be increasing at a rate of 2.4% per year [1,2]. Globally, 6 in 100,000 males and 3 in 100,000 females are diagnosed annually with renal cancer, with the incidence estimated to be increasing at a rate of 2.4% per year [1,2] with limited therapeutic management options [3–5]. In the United States alone, over 60,000 individuals were diagnosed with renal cell carcinoma (RCC) in 2018, with an estimated 14,970 deaths resulting from the illness [6]. Despite the introduction of systemic targeted therapies, five-year survival rates for locally advanced and metastatic disease remain at 70% and 12%, respectively in the year 2020 [7].

The most common genetic abnormality for clear cell RCC (ccRCC) is the chromosome 3p deletion and inactivation of the von Hippel Lindau (VHL) tumor suppressor gene, present in almost all familial and up to 60% of sporadic RCCs [8,9]. Loss of the VHL gene leads to the upregulation of hypoxia-inducible factor (HIF) and activation of vascular endothelial growth factor receptors (VEGFR) and other signaling pathways, leading to tumorigenesis with an aggressive angiogenic phenotype [8,9]. Current treatment of RCC is based on the tumor stage at diagnosis. Localized disease, with or without evidence of regional spread, is typically managed surgically alone. For those with more advanced localized or loco-regional disease after nephrectomy, treatment with adjuvant VEGFR TKI sunitinib has been approved by the Federal Drug Administration, although the modest clinical benefit and concern for the potential side effects has largely limited its clinical application to data [10].

For metastatic ccRCC, a number of approaches combining immune checkpoint inhibitors or with VEGFR-TKIs have now become the standard of care after demonstrating a definitive survival benefit in the first-line setting [11], compared to sunitinib alone [2]. However, the optimal first-line and sequence of subsequent therapies are not well defined. The current systemic therapeutic approaches include: targeting pathways of angiogenesis, immune checkpoint blockade, and mTOR inhibition. Inevitably, treatment resistance is either intrinsic or eventually develops. New mechanistic approaches are urgently needed to improve survival outcomes in this patient population.

Investigative efforts from our group and others have focused on the role of quinazoline-derived α1-adrenoreceptor antagonists in the treatment of renal cancer. By inducing smooth muscle relaxation and vasodilation, these drugs are currently used in the treatment of hypertension (HTN) and renal and ureteric stones [12,13], as well as of benign prostatic hyperplasia (BPH) [13]. More recently the potential efficacy of these drugs in the treatment of prostate cancer has been proposed, due to the ability to induce apoptosis and overcome anoikis resistance in tumor cells [14–18]. Reassuringly, in earlier clinical trials assessing their use for the treatment of BPH, they were well tolerated with only reversible adverse effects in a minority of patients, including postural hypotension (4%), asthenia or light-headedness (10%), somnolence (3%) and retrograde ejaculation (8%) [19,20].

Expression and distribution of α1-adrenoreceptors has been found in the cortex, pelvis, calyces, blood vessels and tubules of the kidney, suggesting potential effects of α1 adrenoceptor antagonists in renal pathophysiology [21–25]. Evidence at the cellular level suggests that the antitumor effect of α1-adrenoreceptor antagonists in renal tumors proceeds via reducing vascularity and impairing growth within the tumor microenvironment (via apoptosis and overcoming anoikis resistance). In this review, we outline the mechanism of α1-adrenoceptor antagonists in targeting renal cancer epithelial and endothelial cells and the potential therapeutic efficacy of using these clinically used FDA-approved drugs for the treatment of advanced RCC.

#### **2. Mechanism of Action of** α**-Adrenoreceptor Antagonists in Human Disease**

Adrenergic receptors (adrenoreceptors) are G-Protein coupled-receptors that are distributed throughout the body. They serve as receptors for catecholamines (noradrenaline and epinephrine) secreted from the autonomic sympathetic nervous system and play an important role in the regulation of a wide range of physiological systems in the body [23,24]. Alpha (α) receptors mediate smooth muscle contraction and vasoconstriction, while beta (β) receptors mediate vasodilation, smooth muscle relaxation, bronchodilation, and excitatory cardiac function [17,21]. The α-adrenoceptors are divided into two classes: α1 and α2, both of which are present in the renal vasculature and mediate vasoconstriction of exogenous and endogenous noradrenaline [22,24]. The α1-adrenoceptors are

further sub-divided into α1A, α1B, α1D, with α1A subtype of therapeutic interest because of its location in the prostate, vas deferens, and urethra in humans [21,23–25].

Quinazoline-derived compounds blocking α1-adrenoreceptors have been found to reduce prostatic smooth muscle tone and relieve overall obstruction, as seen by their success in treating BPH [19]. This mechanism of action is also utilized in the treatment of renal and ureteric stones, as α1 blockers reduce intra-ureteral pressure and increase fluid passage [12,26–28]. Remarkably in human prostatic disease, these compounds not only target the alpha1-adrenergic-receptor mediated smooth muscle contraction [29], but they can also effectively induce apoptosis of tumor epithelial and endothelial cells [17,30,31]. It is important to note that the quinazoline-derived compounds can induce apoptosis among benign prostate epithelial cells, as well as in both androgen-dependent and castration-resistant prostate cancer cells, via α1-adrenoceptor –independent mechanisms [30–35]. This supports a strong cellular basis for their pharmacologic use in other cancer types.

The signaling mechanisms driving the intracellular antitumor action by quinazoline-based α1-adrenoceptor antagonists against prostate cancer epithelial and endothelial cells are summarized on Figure 1. (1) Smad activation of transforming growth factor (TGF)-β1 signaling, which controls cellular proliferation, differentiation, and apoptosis in human cancers cell including prostate cancer cells [3,30–32]; (2) Engaging the death receptor Fas-associated death domain (FADD)-mediated caspase-8 activation and apoptosis induction [3,32,33]; (3) Inhibition of the VEGF-mediated angiogenesis and Akt survival mechanisms navigating tumor vascularity [3,33,34]; and (4) α1-adrenoceptor antagonists have the ability to block cellular adhesion and invasion by targeting cell-cell interaction and impairing cell tight junctions (and also between epithelial and endothelial cells with the extracellular matrix; ECM), consequentially impacting epithelial–mesenchymal-transition (EMT) to mesenchymal–epithelial-transition (MET) phenotypic interconversions and increasing cellular vulnerability to anoikis (Figure 1) [3,35,36].

**Figure 1.** Biological Mechanisms of Anti-tumor Action of Quinazoline α1-Adrenoceptor Antagonists. Schematic diagram demonstrating the signaling mechanisms potentially targeted by quinazolne α1-andrenoceptor antagonists in attenuating renal tumor initiation and progression to metastasis. Quinazoline based α1-andrenoceptor antagonists influence the interconversion of epithelial– mesenchymal-transition (EMT) to mesenchymal–epithelial-transition (MET) phenotypes by targeting

tight junctions and E-cadherin-mediates cell adherence. Tumor cells succumb to anoikis by disruption of integrin-mediated cell survival via integrin-linked kinase (ILK). Quinazoline based α1-andrenoceptor antagonists induce apoptosis by either tumor necrosis factor (TNF)-mediated Fas-associated death domain (FADD)/caspase 8 activity and DNA fragmentation and/or Smad4 activation and apoptotic gene induction by transforming growth factor (TGF)-β. Angiogenesis is inhibited by vascular endothelial growth factor receptor (VEGFR)–tyrosine kinase inhibitors and quinazoline based α1-andrenoceptor antagonists can also target tumor vascularity by disruption of VEGF-mediated HIF1 transcriptional expression and potentially TGF-β signaling.

Moreover we previously established that integrin-linked kinase (ILK), a serine and threonine protein kinase, plays a key role in anoikis resistance by interacting with the cytoplasmic domains of β1-integrin and β3-integrin, which are pivotal in regulating cell adhesion, fibronectin–ECM assembly, and anchorage-dependent cell growth [34–37]. Within the tumor microenvironment (TME), ILK is activated in its phosphorylated form by focal adhesion kinase (FAK) and phosphatidylinositol 3-kinase (PI3-kinase)/Akt pathways [33,38–40]. By inhibiting ILK, quinazoline-derived α1-adrenoceptor antagonists can disrupt these cell-survival signals towards anoikis induction [3,41,42]. Considering that resistance to anoikis (and evasion of apoptosis in detached cells) is a primary contributor to cancer metastasis [43,44] and ultimately lethal disease, the ability to overcome this resistance points to a unique therapeutic value of quinazoline-derived α1-adrenoreceptor antagonists.
