Usefulness of pharmacokinetic/efficacy analysis of an investigational kisspeptin analog, TAK-448, in quantitatively evaluating anti-tumor growth effect in the rat VCaP androgen-sensitive prostate cancer model

Abstract

TAK-448 is a kisspeptin analog with improved in vivo potency. In our previous studies in the rat JDCaP prostate cancer model, TAK-448 showed more rapid and profound reductions in plasma testosterone (T) and prostate-specific antigen (PSA, a biomarker of prostate tumor growth) levels than the gonadotropin releasing hormone (GnRH) analog leuprolide (TAP-144); however, its effects on tumor volume and subsequent tumor recurrence have not been elucidated fully. To overcome these challenges, we established the rat VCaP subcutaneous xenograft model replicating both the androgen-sensitive and castration-resistant phases of prostate cancer, and we performed pharmacokinetic/efficacy (PK/E) correlation analyses to compare the overall anti-tumor growth effects of TAK-448 to those of TAP-144. Our approach demonstrated TAK-448 had greater anti-tumor growth potential, including in the castration-resistant phase, than TAP-144 in this rat VCaP model. TAK-448 treatment was associated with a reduction in intra-tumoral dihydrotestosterone levels, which might explain its superior anti-tumor activity. Thus, our PK/E analysis was effective at providing new insights into the therapeutic efficacy of TAK-448 as a novel ADT agent in our rat VCaP model.

Introduction

Prostate cancer is the third leading cause of death among men in the U.S. (Siegel et al., 2017). Most prostate cancer is diagnosed as a localized early stage tumor, and therapy with curative intent with either surgery or radiation, or both, is usually selected as the primary treatment option (Cornford et al., 2017). However, patients presenting with biochemical failure following treatment with curative intent or with locally advanced or metastatic tumors at diagnosis are normally advised to receive androgen deprivation therapy (ADT) represented by gonadotropin-releasing hormone (GnRH) analog (e.g., leuprolide or TAP-144). Because the majority of prostate cancer shows androgen-sensitive tumor growth (androgen-sensitive prostate cancer or ASPC), ADT achieves tumor shrinkage (Cornford et al., 2017). However, ADT is often followed by tumor recurrence as castration-resistant prostate cancer (CRPC) (Hellerstedt and Pienta, 2002).

Kisspeptin is a 54 amino acid peptide encoded by the KISS1 gene, which was originally known as a metastasis suppressor gene (Ohtaki et al., 2001). Kisspeptin was purified from human placenta and identified as the endogenous ligand for KISS1R (also known as GPR54) (Ohtaki et al., 2001). Subsequent genetic and physiological studies revealed that the kisspeptin/KISS1R system is a key regulator of the hypothalamic–pituitary–gonadal (HPG) axis (Dhillo et al., 2005, Gottsch et al., 2004, Messager et al., 2005). KISS1R is expressed in the hypothalamic GnRH neurons (Messager et al., 2005). When kisspeptin is administered, hypothalamic GnRH neurons are stimulated, leading to the secretion of GnRH from them, followed by luteinizing hormone (LH)/follicle-stimulating hormone (FSH) secretion from the pituitary, and finally the secretion of sex steroid hormones from the gonads (Matsui et al., 2004, Ohkura et al., 2009, Seminara et al., 2003).

TAK-448 is a kisspeptin analog with improved biological stability and in vivo potency compared with natural kisspeptin (Asami et al., 2013). A single injection of TAK-448 induces robust elevations in plasma gonadotropin and testosterone (T) levels in male rats and men; chronic administration, on the other hand, results in profound T reduction (MacLean et al., 2014, Matsui et al., 2012). Compared with TAP-144, TAK-448 was reported to achieve more rapid and profound reductions in T in male rats (Matsui et al., 2012). In accordance with the T reductions, TAK-448 treatment showed also more rapid reduction in plasma prostate-specific antigen (PSA) levels in an ASPC model, JDCaP (Kimura et al., 2009, Matsui et al., 2014). While the JDCaP grew well when transplanted under the subrenal capsule, the tumor could not be maintained as a subcutaneous xenograft in rats, although the reason for this is unknown. Therefore, these previous studies using the JDCaP as a model had limitations in terms of a more precise evaluation of therapeutic benefits. Specifically, these limitations included the lack of an objective measurement of the tumor volume due to the xenograft being located subrenal capsule, and the long time (>6 months) required for the tumor to become castration-resistant, leading to considerable variations between individual animals even in the same treatment group (Tanaka et al., 2018).

To overcome these issues, we established a rat VCaP xenograft model. VCaP is a prostate cancer cell line derived from vertebral metastasis of a patient with hormone-refractory prostate cancer (Labrie, 2011). VCaP cells are categorized as ASPC because they express wild-type androgen receptor and their growth as well as the secretion of PSA is positively regulated by androgens, while they are also capable of proliferating even under low-androgen conditions (Labrie, 2011). In the xenograft model, VCaP tumors initially regress in response to androgen withdrawal (ASPC phase), but soon become refractory to androgen deprivation and resume proliferation (CRPC phase). Moreover, VCaP cells harbor the TMPRSS2:ERG fusion gene, which is observed in 45–65% of clinical prostate cancer tissues. Because TMPRSS2 is an androgen receptor-regulated gene and ERG is known as an oncogene, TMPRSS2:ERG fusion leads to androgen signal-dependent ERG expression. Since these phenotypes resemble clinical androgen-responsive prostate cancer, we considered that the VCaP xenograft model is suitable to recapitulate the nature of both ASPC and CRPC in a relatively short period, and compared the efficacy of TAK-448 and TAP-144 as ADT agents by applying pharmacokinetic (PK) and efficacy (E) correlation analyses. Our data show the usefulness of the PK/E analysis in understanding the therapeutic potential of TAK-448, and we also discuss the possible mechanisms to explain this potential.