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Research Article

Cancer Research Frontiers. 2016 Sept; 2(3): 330-351. doi: 10.17980/2016.330

Clinical pharmacology profile of an oral selective androgen receptor down-regulator, AZD3514: Implications on the design of ongoing castrate-resistant prostate cancer clinical studies

Angela W. Dymond1, Marc-Antoine Fabre1, Gareth D. James2, Masako Hirata3, Simon A. Smith4, Paul A. Dickinson1,5, Michael Dymond6, Glen Clack7

 

1Quantitative Clinical Pharmacology, Early Clinical Development, AstraZeneca, Alderley Park, Macclesfield, UK

2 PHASTAR, Unit 2, 2a Bollo Lane, London W4 5LE, UK

3Clinical Science Division, Research and Development, AstraZeneca K.K., Osaka, Japan

4Oncology Translational Medicine Unit, AstraZeneca, Melbourn Science Park, Melbourn, Royston, UK

5Current Address, Seda Pharmaceutical Development Services®, The BioHub at Alderley Park, Alderley Edge, UK

6Discovery Sciences Statistics, AstraZeneca, Alderley Park, Cheshire, Macclesfield, UK

 7Oncology Translational Medicine Unit, AstraZeneca, Alderley Park, Macclesfield, UK

 

*Corresponding author: Simon A. Smith, Oncology Translational Medicine Unit, AstraZeneca, Melbourn Science Park, Melbourn, Royston, UK. E-mail: Simon.A.Smith@astrazeneca.com; Telephone: +44 7557 540 988

Citation: Angela Dymond, et al. Clinical pharmacology profile of an oral selective androgen receptor down-regulator, AZD3514: Implications on the design of ongoing castrate-resistant prostate cancer clinical studies. Cancer Research Frontiers. 2016 Sept; 2(3): 330-351. doi: 10.17980/2016.330

Copyright: @ 2016 Angela Dymond, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Competing Interests: All authors were at the time of study conduct employees of AstraZeneca with the exception on Gareth D. James, who received payment from AstraZeneca for services rendered. P. Dickinson owns shares in AstraZeneca and is Director of a company with a contract to provide services to AstraZeneca.

Received June 7, 2016; Revised Aug 17, 2016; Accepted Aug 15, 2016. Published Sept 16, 2016

 

Abstract

Purpose: To describe the pharmacokinetics (PK) and pharmacodynamics (PD) of AZD3514 and how the design of the first-time-in-human study was adapted based on the emerging clinical PK.

Patients and methods: Data were collected from 77 patients with castrate-resistant prostate cancer from two dose-escalation studies, in Europe (NCT01162395) and Japan (NCT01351688). PK parameters were derived from plasma and urine data and exploration of PK-PD relationships were performed. Post hoc analysis was conducted to investigate time-dependent changes and inter- and intra-patient variability in PK.

Results: AZD3514 was rapidly absorbed and plasma levels declined in a bi-phasic manner with no ethnic differences. Plasma exposure to AZD3514 was dose proportional. Generally, overall exposures were similar between visits within each patient, but varied between patients within each cohort. A switch to twice-daily dosing, to increase exposure, produced a marked time-dependent reduction in area under the curve of 30% and an increase in apparent clearance (from 17 to 25 L/h) at steady state compared to single doses. Emerging study data showed that low baseline testosterone may influence prostate-specific antigen (PSA) reductions by AZD3514. Combination cohorts with abiraterone acetate, a drug that decreases testosterone in CRPC patients, did not result in meaningful decreases in PSA.

Discussion and conclusions: Despite adaptation of the clinical strategy from emerging PK and PD data, the hypothesis around androgen receptor (AR) modulation through AR down-regulation could not be tested due to the time-dependent effect on AZD3514 PK, which prevented coverage above the target concentration. Further testing of this hypothesis is warranted.               

Key words: clinical trial, phase 1, pharmacokinetics, pharmacodynamics, AZD3514, castrate-resistant prostate cancer, adaptive design

 


Introduction 

Prostate cancer is a common disease and is most often effectively treated with surgery or radiotherapy. However, in patients with aggressive forms or metastatic disease, treatment is more difficult. In such patients, the standard of care is androgen deprivation therapy achieved by either surgical castration or administration of luteinizing hormone-releasing hormone agonists (1, 2). However, the positive effects of androgen deprivation therapy are only temporary and in almost all patients the cancer inevitably returns. Evidence has accumulated that tumor growth in castrate-resistant prostate cancer (CRPC) is still dependent on the androgen receptor (AR) and the AR has become a target for drug development with abiraterone and enzalutamide recently reaching the market (3-6). Unfortunately, despite castrate levels of testosterone in CRPC patients, resistance ultimately develops, which in many cases is still dependent on AR (7-10), and new drugs that are less prone to, or can over-come, the development of resistance are needed.

AZD3514 is a new, first-in class compound that interferes with AR signaling, by binding to AR, inhibiting its nuclear translocation, and ligand-dependent and independent transcriptional activity (11). In the Hershberger rat model (12), AZD3514 caused a dose-dependent decrease in seminal vesicle weight and a reduction of AR protein expression in ventral prostates of castrated adult rats dosed with testosterone propionate (11). AZD3514 also inhibited the growth of androgen-dependent Dunning R3327H prostate tumors in adult rats (AstraZeneca, data on file). AZD3514 differs from other drugs directed at the AR receptor such as enzalutamide and bicalutamide in that it also induces AR down regulation (11).

 AZD3514 was developed as an immediate release formulation as a maleate salt (AZD3514 free base has a pKa of 6.2). The solubility of AZD3514 in pH 6.5 phosphate buffer (25°C) is 18 mg/mL, and 24 mg/mL in pH 1.2 simulated gastric fluid. The Biopharmaceutics Classification System (BCS) guidance for AZD3514 is a tentative BCS4 at the doses being investigated in the clinic.

The pre-clinical PK profile showed that AZD3514 was well absorbed in the dog with maximum concentrations achieved at approximately 5 h with a terminal half-life of 5.6 h. Predictions from pre-clinical species suggested that the target exposure of 2410 ng/ml for 18 h in a 24 h period was expected to be achieved in humans at the 500 mg once-daily (QD) dose. In vitro experiments indicated that AZD3514 is metabolized by CYP3A4 but the metabolic turnover was low. Pre-clinical safety assessment identified gastrointestinal effects which were considered dose-limiting in both rats and dogs (AstraZeneca, data on file) but the overall pre-clinical profile warranted testing in man.

Prostate-specific antigen (PSA) is a biomarker for prostate cancer used in diagnosis, for monitoring diseases progression and assessing therapeutic response. Despite some limitations, notably a lack of specificity for prostate cancer, PSA remains the primary clinical pharmacodynamic biomarker available for prostate cancer used in the clinic (13). In this manuscript, pharmacokinetic and pharmacodynamic (PSA) results from two first-time-in-man studies with AZD3514 in patients with CRPC are reported. We describe how the unusual PK characteristics of AZD3514 and emerging PSA biomarker data led to changes in the design of one of the clinical studies with inclusion of cohorts dosed with AZD3514 in combination with abiraterone acetate. We present the challenges that can occur in first-time-in-man studies that are often not predicted from pre-clinical species and how they may be overcome to test novel mechanism of actions in CRPC. Post hoc analysis to investigate the inter- and intra-patient variability in PK in patients who received AZD3514, and explore if dosage influenced the variability is also presented. Due to the wealth of data gathered, the safety and efficacy data have been reported elsewhere (14).

 

Methods

Patient population

Male patients older than 20 years of age with CRPC and a life expectancy of at least 12 weeks could be enrolled. The patients had to have a documented evidence of metastatic prostate cancer for which no standard therapy was considered appropriate. Details of the studies inclusion/and exclusion criteria have been reported previously (14). All patients were required to provide signed and dated written informed consent prior to any study specific procedures.

 

Study design

There were two phase I, open-label, multicenter, dose escalation studies. Study 1 was conducted in 5 centers in the UK, US and The Netherlands, and study 2 was conducted in 2 centers in Japan. All patients in Study 1 were white and of European ancestry. Both studies followed similar methodology but fewer doses were tested in Japan. Both studies were performed in accordance with the Declaration of Helsinki and the International Conference on Harmonization Good Clinical Practice Guidelines and approved by relevant regulatory and independent ethics committees.

At least 3 and up to 6 evaluable patients were required for each dose cohort. However, patient cohorts at selected doses could be expanded to a maximum of 12 patients to investigate further the tolerability, pharmacokinetics and biological activity of AZD3514. Each patient received a single dose of AZD3514 on Day 1 and after a washout of 7 days multiple dosing was started on Day 8 onwards until discontinuation. In study 1, the following single and multiple doses of AZD3514 were investigated: 100, 250, 500 and 1000 mg QD and 1000 and 2000 mg twice daily (BID). Study 2 was opened after the first 2 dose cohorts had been recruited in Study 1. Patients in Study 2 received single doses of 250, 500 or 1000 mg and multiple doses of 250 or 500 mg QD and 500 mg BID. These doses were selected based on findings from Study 1. The study drug was taken in the fasted state (food restriction for at least 2 hours before and 1 hour after administration of AZD3514). In study 1, based on emerging pharmacokinetic/pharmacodynamic modeling results, additional cohorts were included investigating the effects on PSA from 500 mg BID AZD3514 in combination with 1000 mg QD abiraterone acetate.

On study days 1, 8 and 29, venous blood samples for measuring AZD3514 plasma concentrations were taken at the following time points for the once-daily dosing regimens: pre-dose, 0.5, 1, 2, 3, 4, 5, 6, 8, 10, 24 hours, and on Day 1 only: 48, 72 and 96 hours post-dose. Similar pharmacokinetic sampling was performed for the twice daily dosing regimens with the exception that on Days 8 and 29, the 10 h samples were replaced with a collection at 12 h post-dose. Urine collection for pharmacokinetic purposes started immediately pre-dose until 10 h post-dose on Days 1 and 29. PSA levels were measured at screening, on Days 8, 15, 29, 57 and 85 and once every 4 weeks thereafter until discontinuation.

 

Pharmacokinetic analysis including post hoc assessment of temporal change in apparent clearance and inter and intra patient variability in PK

Plasma samples were prepared by solid phase extraction and liquid chromatography. The concentrations of AZD3514 in plasma and urine were quantified by tandem mass spectroscopy. Pharmacokinetic parameters were determined by non-compartmental analysis using Phoenix™-WinNonlin® v6.3. PK parameters were derived by the following methods: maximum observed concentrations (Cmax, Cssmax) and time to maximum concentrations (tmax, tssmax) were determined by inspection of the concentration-time profiles; λz was calculated by log-linear regression of the terminal portion of the plasma concentration-time profiles; terminal half life (t½λz) was calculated as Ln(2)/λz; the area under the plasma concentration-time curve up to the last quantifiable sample (AUC(0-t)), the area under the plasma concentration-time curve up to the end of the dosing interval (AUCtau) and the area under the curve at steady state (AUCss) were calculated using the linear up/log down trapezoidal rule; where appropriate, AUC(0-t) was extrapolated to infinity using λz to obtain AUC0-∞; apparent clearances (CL/F following the single dose and CLss/F following multiple dosing) were determined from the ratio of dose/AUC0- or dose/AUCss. Apparent volume of distribution (V/F) was determined from the mean residence time x CL/F; accumulation ratio was determined from the ratio of AUC(0-t) on Day 29/AUC(0-t) on Day 1; temporal change parameter was calculated from the ratio of AUCtau Day 29/AUC Day 1.

A power model (15) was used to explore the dose proportionality of AZD3514 pharmacokinetics. Possible effects of race on the pharmacokinetics of AZD3514 were analyzed descriptively and explored graphically. AUC0-24h was chosen for this analysis in order to include patients for whom AUC0- could not be calculated. Comparison of individual Day 1 against Day 29 apparent clearance in Western patients was performed post hoc using a 2-sided paired t-test with a 5% significance level, no statistical analysis was performed with the Japanese dose groups due to the low number of patients (n = 4-5 per cohort). The inter and intra patient variability between Days 1 and 8 was assessed in terms of Ln(AUC) and was conducted using SAS version 8.2. All patients who received doses of 100 mg to 1000 mg AZD3514 and had AUC measurements on Days 1 and 8 with no exclusions due to emesis were used in the variability analysis. The correlation between the Ln(AUC) at Days 1 and 8 was calculated for each dose, and a Bland Altman test (16) was conducted to determine any Ln(AUC) time effects.

 

 

fig1

Figure 1. AZD3514 geometric mean single-dose plasma concentration-time profiles. Geometric mean plasma concentration-time profiles of AZD3514 in CRPC patients (n= 5 to 12 per dose group) after administration of single doses of 100, 250, 500 or 1000 mg AZD3514 (linear scale). The insert shows the profiles on a semi-logarithmic scale.

 

Population pharmacokinetic modeling

A retrospective population PK analysis was performed at the end of the studies to evaluate any potential PK dependent covariates and to support the NCA findings.

This data was not used for decision making when the studies were ongoing. The population PK analysis was based on multiple regressions using NONMEM program (version 7.2). Since the data set consisted exclusively of rich data the first-order conditional estimation (FOCE) method performed well for the stability, robustness and predictability of the model.

The model included four basic components as follows: (i) the structural PK model, which predicts plasma concentration as a function of time and dose; (ii) the covariate model component, which describes the influence of fixed effects (e.g. demography, laboratory data) on PK parameters; (iii) the between-subject variance component, which describes the inter-individual variation in PK parameters (after ‘correction’ for fixed effects); and (iv) the residual error model component, which describes the underlying distribution of the error in the measured plasma concentrations.

The selection of the structural PK model and variance models for residual error was based on the goodness-of-fit plots and on the difference in NONMEM objective function (-2LL: – 2xLog Likelihood) between hierarchical models (i.e. the “likelihood ratio” test). The covariate models used in this analysis represent shifts from the “typical” subject parameter value. Potential covariates were selected by univariate analysis, testing the effect of each covariate on each of the relevant PK parameters. A p value of 0.05 was chosen to retain one parameter, i.e. a difference in the objective function ≥ 3.84 for one degree of freedom. The covariates identified by the univariate selection were then included into a “full” model after ranking by the size of change in the objective function; rank 1 having the largest influence on the objective function. The covariates evaluated in this study were race, age, bodyweight, body mass index, body surface area, creatinine clearance, alanine and aspartate aminotransferases and bilirubin.

Population PK models were acceptable if they resulted in successful minimization and a successful estimation of the covariance. Other requirements were that 95% confidence intervals of structural parameters should not include zero, and the absolute value of correlation between two structural parameters should not be >0.95. Diagnostic plots (population prediction versus observed concentrations, individual prediction versus observed concentrations, weighted residual versus population prediction and weighted residual versus time after dosing) were used to evaluate the goodness-of-fit throughout the model-building procedure.

 

Results

The patient demographics, clinical characteristics and details of prior anti-tumour therapies have been reported previously (14). All patients had metastatic disease with 84% having metastasis to bone, 49% metastasis to lymph nodes and 12% metastasis to viscera.

Pharmacokinetics of QD dosing

AZD3514 concentration-time profiles after single-dose administration in study 1 are shown in Figure 1. The pharmacokinetic parameters from the non-compartmental analysis from both studies following a single dosing and at steady state are summarized in Tables 1 and 2 respectively. Plasma concentrations of AZD3514 peaked at 2 to 2.9 hours with no apparent lag in absorption, after which, concentrations declined in a bi-phasic manner at all doses in both studies and the majority of AZD3514 (>90% of AUC0-) was cleared by 24 hours post-dose at all dose levels. The elimination phase was characterized by a t1/2 of approximately 16 h in study 1, which appeared independent of dose (Table 1). At steady state on Day 29 and following QD dosing, values for Cssmax and tssmax were generally similar to those observed after single-dose administration and there was no evidence of accumulation (Tables 1 and 2). Increases in the mean apparent clearance at steady state (CLss/F) of ~7% to 31% at 100 to 1000 mg QD compared to single dose administration was detected, resulting in a small time-dependent effect on AUC with mean temporal parameter change values ranging from 0.85 to 0.91.

Plasma exposure to AZD3514 increased proportionally with dose after both single- and multiple-dose administration in both Western and Japanese patients (Table 3). The inter-patient variability, based on group % CV, was low to moderate ranging from 1.3% to 44% for AUC0-∞ after single AZD3514 doses (Table 1) and 8.9% to 35% for AUCtau on Day 29 (Table 2).

The post hoc variability assessment included 35 patients, 7 patients were excluded due to emesis and two patients were excluded as their PK measurement was missing at either Day 1 or day 8. The results showed that for intra-patient variability, ln(AUC) between Day 1 and 8 in the AZD3514 monotherapy cohorts was highly correlated in the 100 mg, 250 mg and 500 mg QD cohorts (90, 75 and 95% respectively), and moderately correlated in the 1000 mg QD cohort (41%). A Bland Altman test indicated there was no evidence of a time effect on ln(AUC) (95% confidence interval: -0.055, 0.078). Hence, ln(AUC) values were similar on Days 1 and 8 for the majority of patients indicating low intra-patient variability. The ln(AUC) varied more between patients at the same dose than within patients between Day 1 and 8, but overall the observed inter-patient variability was modest for a compound metabolized exclusively by CYP3A4 and with low renal clearance. Post hoc analysis results are shown in Supplementary Table 1 and Supplementary Figure 1. In all the above analyses, one patient from the 100 mg cohort in study 1 was excluded from the descriptive statistics as he was later found to have taken the prohibited co-medication diltiazem, a moderate CYP3A4 inhibitor (17), which has the potential for PK interaction with AZD3514. In the 1000 mg QD cohort in study 1, one patient’s Day 29 results were excluded due to a late dose administered on Day 28 which resulted in increased AZD3514 plasma concentrations on Day 29.

 

Change to twice-daily dosing to increase exposure

The exposure of AZD3514 at 1000 mg QD dosing did not reach the target coverage of 2410 ng/mL for 18 hours in a 24 h period, and simulations suggested that escalation to 2000 mg QD would not reach the desired threshold either. The inability of QD dosing to reach the target coverage lead to a change to twice-daily (BID) dosing beginning at the 1000 mg dose.

 

 

Table 1. Single-dose pharmacokinetic variables of AZD3514 in Western (study 1) and Japanese (study 2) CRPC patients

table1

Data are expressed as geometric mean (%CV) for AUC and Cmax, median (range) for tmax, and arithmetic mean (±SD) for t1/2, CL/F and V/F

QD, once daily; BID; twice daily; AUC0-, area under the curve from time zero to infinity; Cmax, maximum concentration; tmax, time to maximum concentration; t1/2, terminal elimination half-life; CL/F, apparent clearance; V/F, apparent volume of distribution

 

A marked temporal change in the pharmacokinetics of AZD3514 was apparent at 1000 mg BID following multiple dosing, with a 30% lower overall exposure on Day 29 compared to Day 1 (individual temporal parameter change values ranged from 0.57 to 0.76) (Table 2). Post hoc statistical comparison of individual changes in apparent clearance from Day 1 to Day 29 (Figure 2) indicate no marked differences at 100 to 500 mg QD of AZD3514, although most patients showed some increase with multiple dosing. Statistically significant increase in CLss/F on Day 29 at 1000 mg QD and 1000 mg BID compared to CL/F on Day 1 was detected, however, the small group sizes and the magnitude of change with respect to clinical relevance should be taken into consideration. All patients dosed at 1000 mg BID showed increased CLss/F with a clinically relevant increase of ~50% in the group mean (17.1 L/h on Day 1 to 25.0 L/h on Day 29) which would warrant a dose alteration. Consequently, a further dose escalation at 2000 mg BID was tested.

Doses of 100 to 1000 mg QD and 1000 mg BID were well tolerated, however, on escalation to 2000 mg BID the occurrence of non-tolerated nausea and vomiting resulted in the discontinuation of dosing prior to Day 29, hence no steady state data were obtained. The switch to BID dosing did not permit attainment of the desired target coverage, and the clinical strategy changed to consider alternative treatment strategies.

 

Renal excretion

The mean fraction of AZD3514 excreted in urine (fe) ranged from 2.6 to 6.6% of dose resulting in low renal clearance (CLR), ranging from 0.45 L/hour to 1.17 L/hour. Similar renal excretion was detected following single and multiple dosing and was independent of dose and schedule. At the 1000 mg BID dose when marked temporal change in PK was detected, the fe and CLR remained low and were similar following single and multiple dosing (fe at 3.8% and 2.2%; CLR at 0.88 and 0.54 L/hour, on Day 1 and Day 29, respectively), so although overall clearance increased over time, renal clearance was not affected.

 

Table 2. Steady state pharmacokinetic variables of AZD3514 in Western (study 1) and Japanese (study 2) CRPC patients

table2

Data are expressed as geometric mean (%CV) for AUC, Cssmax and Cssmin, median (range) for tssmax, and arithmetic mean (±SD) for CLss/F, accumulation ratio, temporal change parameter and time above target concentration in 24-hour period

QD, once daily; BID; twice daily; AUCtau, area under the curve during a dose interval; Cssmax, maximum concentration at steady state; tssmax, time to maximum concentration at steady state; Cssmin, minimum concentration at steady state

 

 

fig2

Figure 2. Individual changes in apparent clearance from Day 1 to Day 29. Comparison of individual changes in apparent clearance from Day 1 to Day 29 indicate a marked temporal change in the pharmacokinetics of AZD3514 that was apparent at 1000 mg BID following multiple dosing.

 

 

Impact of ethnicity

Comparison of the systemic exposure of AZD3514 between Japanese and Western patients at equivalent dose levels showed only minor ethnic differences with Cmax and AUC0- appearing marginally higher in Japanese patients at the 500 and 1000 mg QD doses (Table 1). The mean bodyweight of Japanese patients was approximately 17% lower than that of the Western patients, so when AUC0-24h was normalized for body weight and dose, no difference in exposure was apparent (Figure 3).

 

Population pharmacokinetic modeling

A two-compartment linear model with zero order absorption was found to adequately describe the AZD3514 plasma concentration-time profile. The time dependency on pharmacokinetic parameters was assessed as prior information during the development of the structure model (base model). The models used to assess the time effect on PK parameters were a categorical and saturation model (Emax or Hill model). The results of the model evaluation showed statistically significant changes in the objective function, and parameters were well estimated for the simple categorical model on CL/F and V2/F. The time dependency was also tested on bioavailability (F), in the same manner as described aforesaid; however the statistical change was minor and thus not selected in the structure model. During the covariate analysis, there were no correlations between apparent clearance, apparent initial volume or apparent peripheral volume and the race, age, weight, body mass index or creatinine clearance of the patients. Taking into account these covariates, the PK exposure from the population-PK model was found to be the same between Western and Japanese patients confirming the conclusion derived from the non-compartmental analysis. The parameters of the final model are provided in Table 4. The diagnostic goodness-of-fit visual predictive check (VPC) plots (Supplementary Figure 2), show no bias in the population or individual predictions. The plots of conditional weighted residuals vs. predictions and time show that most residuals were small and evenly distributed between –2 and 2 (data not shown), and therefore demonstrate the strong predictivity of the model.

 

 

fig3

Figure 3. Effect of race on AZD3514 exposure. The figure shows individual single-dose body weight and dose-normalized AUC0-24h for Western and Japanese patients. When AUC0-24h was normalized for body weight and dose, no difference in exposure was apparent.

 

Effects of AZD3514 on PSA

The effect of different doses of AZD3514 on the level of PSA during the first two months of treatment in Western CRPC patients is shown in Figure 4. The 57-day time point was chosen as the cut off for this graphical analysis because most patients were treated for at least 57 days whereas treatment discontinuations were frequent thereafter. The mean baseline PSA value varied from 137 to 478 ng/mL between dose groups (Figure 4), whereas individual baseline PSA values varied from 6.5 to 5408 ng/mL (data not shown). No apparent effects of AZD3514 on the levels of PSA were noted based on graphical examination (Figure 4). However, in some patients (12 out of 70 in the monotherapy groups), treatment with AZD3514 resulted in a clinically interesting (>30%) transient decrease in circulating PSA within 12 weeks of treatment (Figure 5), suggesting that modulation of AR signaling was occurring albeit sub optimally. Similar observations were found in Japanese patients (study 2, data not shown). To explain these observations, a systems pharmacology (mechanistic) modeling approach was undertaken to determine if AZD3514 may be effective in a specific sub-population of patients, with the expectation that it might help inform the ongoing clinical study design (18).

The systems pharmacology model indicated AZD3514 may be more effective in a low dihydrotestosterone (DHT) environment and so administration of AZD3514 with abiraterone acetate (which blocks the production of DHT) may result in greater efficacy even at lower exposures of AZD3514 (18). As target exposure for efficacy was not achieved by AZD3514 alone, this alternative dosing strategy may potentially provide a viable treatment option. Thus, abiraterone combination cohorts dosed at 500 mg BID AZD3514 (a proposed long-term tolerated dose) were opened, initially in abiraterone acetate naïve patients and later in patients on abiraterone acetate at the time of progression. Adjustment of the patient population was made owing to this population being identified as an area of unmet medical need and that evidence of resistance to abiraterone acetate provided an opportunity to demonstrate the benefit of adding AZD3514. One of the 5 patients that had progressed on abiraterone and was treated concomitantly with 500 mg BID AZD3514, had a much higher baseline PSA value than the other 4 patients and this patient discontinued treatment within 2 weeks of coming onto study. In the remaining 4 patients, AZD3514 did not lower PSA levels.

 

Table 3. Dose proportionality of AZD3514 pharmacokinetics

table3NC = not calculable

 * AUC, area under the curve (AUC0- Day 1; AUCtau at steady state for day 29)

 

 

Discussion

Challenges in the clinical drug development of AZD3514

The unexpected clinical pharmacokinetics of AZD3514 and significant issues with long-term tolerability, namely nausea and vomiting (14) at high doses, presented challenges to the development of AZD3514 resulting in adaptive changes to the design of the clinical studies (Figure 6).

In CRPC patients, the pharmacokinetic profile of AZD3514 is characterized by 1) rapid absorption with tmax values of about 2 h, 2) biphasic disposition with a terminal t1/2 of about 16 h, 3) dose proportionality in the once daily dose range investigated, 4) no accumulation after QD and BID dosing, 5) low renal excretion of parent compound and 6) marked time-dependent pharmacokinetics apparent at higher doses and shorter dosing interval. The apparent discrepancy between a t1/2 of about 16 h and the lack of accumulation may be explained by a first elimination phase accounting for the majority of the elimination of AZD3514. The combination of a lack of accumulation and time-dependent pharmacokinetics resulted in low trough concentrations that were insufficient to achieve the target drug levels over a specified time predicted to be required for efficacy. The underlying mechanism for the marked reduction in AZD3514 exposure after multiple-dose administration compared to single doses is unknown. One possibility is that AZD3514 activates mechanisms that facilitate its own metabolism by induction of CYP3A4 since AZD3514 is almost exclusively metabolized by this route.

However, in vitro studies indicate that AZD3514 does not activate the pregnane X receptor, a nuclear receptor involved in the regulation of a wide variety of genes involved in the elimination of xenobiotics and activated by known inducers of CYP3A4 (19, 20). Furthermore, in cultured human hepatocytes, AZD3514 did not induce CYP1A1/2, CYP2B6 or CYP3A4 at concentrations up to 100 µM (AstraZeneca, data on file), which is well above clinical exposures. These data suggest that auto induction of metabolizing enzymes by AZD3514 is unlikely but this cannot be ruled out. To determine whether hepatic CYP3A4 enzyme induction may be occurring in the clinic, measurement of the biomarker, 4β-hydroxycholesterol, was planned for new patients recruited onto study 1. However, the study was stopped before samples could be collected and thus there is no clinical data to support the potential for auto induction of metabolizing enzymes by AZD3514. As a point of note, the measurement of 4β-hydroxycholesterol requires the collection of only a few blood samples and incurs limited cost, and thus the early inclusion of 4β-hydroxycholesterol measurements to evaluate clinical enzyme induction is worth consideration in first-time-in-man studies. Alternative in vivo mechanisms of actions such as changes in absorption and hence bioavailability with time could not be ruled out as these were not investigated.

 

Table 4. Final population pharmacokinetic parameters.

table4

CI, confidence interval; CV%, coefficient of variation; IIV, inter-individual variability; TDD, total daily dose; CL/F, apparent clearance; V1/F, volume of the central compartment, V2/F, volume of the peripheral compartment;

 

Atypical low AZD3514 variability

An exploratory investigation showed that multiple oxidative metabolites were present in the blood samples with parent constituting about 40% of overall drug related material at 3 h post dose (AstraZeneca, data on file). Clinical evidence that CYP3A4 is substantially involved in the in vivo metabolism of AZD3514 comes from the pharmacokinetic results of the one patient who was co-administered with a moderate CYP3A4 inhibitor, diltiazem, in error. In this patient, the overall exposure (AUC) to AZD3514 was about two-fold higher than the other patients with minimal effect on Cmax. This suggests a small first-pass extraction of AZD3514 which is supported by the moderate CL/F of 13 to 17 L/h in western patients which is ~ 17% of liver blood flow. Although Ln(AUC) measurements varied between patients on the same doses, values on Day 1 and 8 for each patient were generally similar. However, considering the apparent substantial contribution of CYP3A4 to the clearance of AZD3514 the variability was rather low with a CV generally below 30%. Based on the expectation that CYP3A4 is the main enzyme involved in the clearance of AZD3514, the PK profiles and particularly the variability observed are somewhat surprising. The PK of compounds that are thought to be exclusive substrates of CYP3A4 have been investigated extensively in an attempt to identify a probe substrate that can be used to profile the CYP3A4 capacity of an individual to allow dose individualization for all CYP3A4 substrates. Benet (21) and Masica et al. (22) investigated the three benzodiazepines, alprazolam, triazolam and midazolam all of which are metabolized exclusively by CYP3A4 and not subject to p-glycoprotein efflux. They reported mean oral clearance of 4.5, 28 and 92 L/h for alprazolam, triazolam and midazolam with a % standard deviation of 48, 42 and 50% respectively. The mean oral clearance of AZD3514 in the dose proportional pharmacokinetic dose range of 100 to 1000 mg was ~ 15 L/h which falls within the range of clearance observed by Masica and co-workers. Other workers have reported high % standard deviations when measuring the clearance of CYP3A4 substrates such as 73% and 63 % for alfenatil and midazolam respectively (24), and 47% for midazolam (24).

 

 

fig4

Figure 4. PSA responses after treatment with AZD3514. The effect of different doses of AZD3514 on the level of PSA during the first two months of treatment in Western CRPC patients is shown as mean ± standard deviation for each dose cohort.

 

Kato and co-workers (25) developed a method for predicting the inter-individual variability of human exposure for CYP3A4 substrates using Monte Carlo simulation. Using this model they concluded that inter-individual variability was related to clearance with higher clearance drugs (fh > 0.38) resulting in higher %CV. The majority of the variability was attributable to CYP3A4 expression rather than variability in other physiological factors such as hepatic blood flow. To achieve a good fit of the model data to the CYP3A4 substrates selected to assess the model against, Kato and co-workers had to select the lowest reported %CV for CYP3A4 expression in liver microsomes (33%). This is compared to other reported values that were in the range of 50 to 100%. It is suggested this is because the % CV reported for CYP3A4 microsomal expression might be artificially high as a consequence of recovery or degradation issues during the microsomal preparation. Although another potential explanation maybe that some of the substrates selected are not exclusively cleared by CYP3A4 (for instance: efavirenz (CYP2B6); rapaglinide (CYP2C8); loratidine (CYP2D6); diazepam (CYP2C19)) and consequently the impact of variable CYP3A4 expression is attenuated by the additional clearance pathway. Although Kato et al. show a lower %CV for lower clearance CYP3A4 substrates than the other reports discussed above, the %CV of AZD3514 in patients is still towards the lower end of variability reported by Kato et al for drugs with similar clearance. Based on the literature review, the PK variability of AZD3514 appears to be unusually low considering the relatively strong evidence that CYP3A4 is predominately responsible for AZD3514 clearance.

Ethnicity is a demographic variable that may contribute to inter-individual variability in pharmacokinetics and/or pharmacodynamics of drugs (26). Although ethnic differences in exposure and/or response are not uncommon, in only a few cases have they led to population-specific prescribing recommendations (27). In the present study, it was shown that exposure to AZD3514 was slightly higher in Japanese patients when compared to Western patients; however, this difference was no longer apparent after normalization for body weight. The observed smaller volume of distribution in Japanese patients is consistent with this. Differences exist between Asian and Western patients regarding CYP3A4 liver content (28) but the activity of this and that of other major drug metabolizing enzymes was similar in these populations (29) and, therefore, do not appear to contribute to any observed differences in pharmacokinetics.

 

 

fig5

Figure 5. Effect of AZD3514 on individual PSA responses. The PSA profiles of individual patients are shown which reveal clinically interesting (>30%) transient decreases in circulating PSA within 12 weeks of treatment in some patients.

 

Testing the androgen modulation hypothesis

The introduction of PSA as a biomarker for prostate cancer was an important step forward in the ability to diagnose this disease and offer the patient earlier and more effective treatment (13). Not only is PSA used to diagnosis this disease, but also to monitor disease progress and assess therapeutic response. PSA is the primary clinical biomarker available for prostate cancer that is used outside the purely research environment (13). Unfortunately, treatment with AZD3514 in patients did not result in a dose-dependent and consistent decrease in PSA. From animal experiments it was predicted that in order for AZD3514 to decrease PSA in patients, the plasma concentrations of this compound needed to be above 2410 ng/mL for at least 18 h during a 24 h period. At these exposures of AZD3514 in LNCaP cells, PSA mRNA expression was reduced by 90-100% (30). It became clear that this goal could not be achieved with QD dosing and the dosing frequency was increased to BID. However, due to a temporal change in PK, the target coverage was not achieved. Simulations suggested that dose escalating to 2000 mg BID could have provided sufficient coverage for efficacy, but the occurrence of non-tolerated gastrointestinal adverse events lead to discontinuation of 2000 mg BID dosing in patients (14).

Although AZD3514 did not decrease PSA in general, some patients showed clinically significant decreases in PSA (Figure 5). A systems pharmacology modeling approach was undertaken with the expectation that it might help inform the ongoing clinical study design. Initial empirical modeling on the change in PSA over time found that no PK variables, such as the area under the curve, and minimum and maximum plasma concentrations, were strong correlates of the growth constant. Subsequently, other variables were investigated, such as baseline values of markers, co-medications, age, pre-treatment PSA trajectory, and any other variables that had been collected. Counter-intuitively, considering the mode of action of AZD3514, baseline PSA was identified as the only potentially predictive covariate (18). Subsequently a mechanistic model was developed that indicated that AZD3514 may be more active in a low dihydrotestosterone (DHT) environment (18). PSA gene expression is reliably stimulated by androgens such as DHT (31), hence providing a potential hypothesis why low baseline PSA may influence AZD3514 activity. This finding was the basis for investigating a combination drug strategy which involved administration of AZD3514 in combination with abiraterone, a drug which decreases DHT (and consequently PSA) in CRPC patients (32), to cohorts of patients in study 1 (Figure 6). The combination dose for AZD3514 was 500 mg BID as this was expected to be well tolerated for long term treatment and to provide a safety margin as no data were available on the safety of the abiraterone and AZD3514 combination. Recruitment of patients that had progressed on abiraterone proved slow and the mean PSA baseline in this group was no lower than that of the 1000 mg QD AZD3514 monotherapy group (Figure 4). The addition of AZD3514 to the treatment regimen of abiraterone-treated patients did not produce any meaningful decrease in PSA in the 5 patients recruited into the cohort. As a result, an early futility analysis was performed which gave a low probably of achieving target efficacy and the study was stopped. However, it can be argued that the low PSA baseline hypothesis was not fully tested in the clinical study due to the lack of patients recruited with low baseline PSA, the low AZD3514 exposure achieved and the small number of patients recruited. Further exploration of this hypothesis with other drug therapies is worthy of consideration in prostate cancer research.

 

 

fig6

Figure 6. Clinical study design adaptation resulting from emerging study data. The figure shows how the AZD3514 clinical programme was modified in response to study data, in the first instance to achieve the pre-defined target exposure of AZD3514, and in the second instance to initiate combination cohorts with abiraterone when it was suggested that the effect of AZD3514 may be better manifested at low DHT levels.

 

In conclusion, the emerging unexpected clinical PK of AZD3514 and systems pharmacology modeling (18) that suggested greater efficacy may be achieved with low baseline androgen resulted in the transition to an adaptive clinical trial design to explore a drug combination strategy with abiraterone. However, due to the time dependent decrease in exposure which was exacerbated with twice daily dosing, coverage above the target concentration could not be achieved. Nevertheless, the observation that in some patients a clinically meaningful decrease in PSA was observed may indicate that follow-up compounds with a similar mechanism of action but with an improved pharmacokinetic and safety profile may prove to be useful in the treatment of CRPC either as monotherapy or in combination with abiraterone. Additionally, AZD3514 as a CYP3A4 substrate with low variability and time dependent pharmacokinetics that do not seem to be explained by CYP3A4 induction may be an interesting tool compound for academic research that may provide new insights into drug disposition.

 

Acknowledgements

The authors thank AZD3514 study 1 and 2 clinical investigators and patients, Henk Poelman (PRA International, Assen, The Netherlands) for the bioanalytical work and Paul van Giersbergen (Van Giersbergen Consulting, Wuenheim, France) for editorial assistance.

 

ABBREVIATIONS

AR, androgen receptor;

BID, twice daily;

CRPC, castrate resistant prostate cancer;

DHT, dihydrotestosterone;

PD, pharmacodynamic;

PK, pharmacokinetic;

PSA, prostate specific antigen;

QD, once daily

 

Supplementary Figure 1-2

Supplementary Table 1

 

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