Current Medicinal Chemistry, 2005, 12, 277-296
`
`277
`
`Novel Biological Agents for the Treatment of Hormone-Refractory Prostate
`Cancer (HRPC)
`
`A.G. Papatsoris1,2, M.V. Karamouzis1,3 and A.G. Papavassiliou*,1
`
`1Department of Biochemistry, School of Medicine, University of Patras, Patras, Greece
`22nd Department of Urology, School of Medicine, University of Athens, “Sismanogleio” General Hospital, Athens,
`Greece
`31st Department of Medical Oncology, “St. Savvas” Anticancer-Oncologic Hospital, Athens, Greece
`Abstract: Hormone-refractory prostate cancer (HRPC) is an inevitable evolution of prostate carcinogenesis,
`through which the normal dependence on hormones for growth and survival is bypassed. Although advances
`in terms of symptoms palliation and quality of life improvement have been addressed with current treatment
`options, innovative approaches are needed to improve survival rates. A thorough understanding of HRPC-
`associated molecular pathways and mechanisms of resistance are a prerequisite for novel potential therapeutic
`interventions. Preclinical and early clinical studies are ongoing to evaluate new therapies that target specific
`molecular entities. Agents under development include growth factor receptor inhibitors, small molecules
`targeting signal transduction pathways, apoptosis and cell-cycle regulators, angiogenesis and metastasis
`inhibitors, differentiation agents, telomerase inactivators, and epigenetic therapeutics. Incorporation of these
`agents into existing treatment regimens will guide us in the development of a multidisciplinary treatment
`strategy of HRPC. This article critically reviews published data on new biological agents that are being tested
`in HRPC clinical trials, highlights ongoing research and considers the future perspectives of this new class of
`agents.
`Keywords: Biological agents, Gene therapy, Hormone-refractory prostate cancer, Immunotherapy.
`
`INTRODUCTION
`
`Prostate cancer remains the most common non-cutaneous
`malignancy in the Western world and is the second leading
`cause of cancer death in males, after lung cancer [1]. In 2002,
`nearly 189,000 men received a diagnosis of prostate cancer in
`the United States and there were an estimated 30,200
`prostate cancer-related deaths [2]. Autopsy series have
`revealed small prostatic carcinomas in up to 29% of men 30
`to 40 years-old and 64% of men 60 to 70 years-old [3].
`Moreover, prostate cancer risk is 1 in 6 and death risk from
`metastatic disease is 1 in 30 [2]. Unfortunately, localised
`prostate cancer rarely causes symptoms, thus 38 to 51% of
`patients present with locally advanced or metastatic disease,
`while 10% to 50% of these cases will rapidly progress to a
`hormone-refractory state [4].
`Despite these grim statistics, surprisingly little progress
`has been achieved in extending patients’ survival with
`current treatment modalities. Noteworthy is that since the
`first observation concerning the beneficial effects of castration,
`by Huggins and Hodges in 1941, androgen ablation still
`remains
`the cornerstone of advanced prostate cancer
`treatment. Although tumour regression is initially achieved
`in the majority of patients, progression to hormone-refractory
`prostate cancer (HRPC) usually occurs within 2 to 5 years
`[5]. HRPC current therapy is mainly directed at palliation of
`symptoms and improving the quality of life, offering 7 to 16
`
`the Department of
`this author at
`to
`*Address correspondence
`Biochemistry, School of Medicine, University of Patras, GR-26110, Patras,
`Greece; Tel: +30 2610 996144; Fax: +30 2610 996110; E-mail:
`papavas@med.upatras.gr
`
`0929-8673/05 $50.00+.00
`
`months median survival [6,7]. Ongoing research explores in
`depth
`the molecular mechanisms
`implicated
`in
`the
`emergence of hormone independence in prostate cancer [8].
`Based on experimental and preclinical findings, novel anti-
`prostate cancer strategies have been developed. The present
`review focuses on the rationale of novel biological agents and
`strategies, which are evaluated for the treatment of HRPC
`and considers their future perspectives.
`
`1. HRPC DEFINITION
`
`Prostate cancer represents a heterogeneous entity, with
`both hormone-sensitive and hormone-insensitive cells
`present since
`initial diagnosis [9]. HRPC
`refers
`to
`progressive disease despite castration levels of testosterone.
`Androgen ablation can usually inhibit the progression of
`endocrine-sensitive prostate cancer cells. However, some
`cells continue to proliferate despite castration levels of
`testosterone and remain sensitive to alternative endocrine
`treatments such as adrenal-androgen ablation, corticosteroids
`and anti-androgen withdrawal. Noteworthy is that there is
`not a widely accepted definition of HRPC [10]. Recently,
`established criteria for patients with HRPC recruited in
`clinical trials require the presence of at least one new lesion
`on bone scan or biochemical progression, in the presence of
`castration levels of
`testosterone (< 50 ng/mL) [11].
`Biochemical progression is considered when two consecutive
`increases in prostate-specific antigen (PSA) are registered,
`with a minimal value of 5 ng/mL. Finally, progression
`occurs following cessation of treatment with androgen
`receptor blockers for 4 to 6 weeks.
`
`
`
`
`JANSSEN EXHIBIT 2042
`Mylan v. Janssen IPR2016-01332
`
`

`

`278 Current Medicinal Chemistry, 2005, Vol. 12, No. 3
`
`2. ANDROGEN RECEPTOR (AR) AND HRPC
`
`Prostate tissue development, growth, differentiation and
`homeostasis depend on androgen activity, mediated by the
`AR which is a member of the steroid hormone receptors’
`superfamily and represents a “zinc-finger” transcription factor
`[12]. The AR is androgen activated upon ligand bonding,
`resulting in dimerisation and recognition of androgen
`response elements, located in the promoter or enhancer
`regions of AR-target genes in the nucleus. Although
`experimental data suggest that before ligand bonding the
`receptor is located in the cytoplasm bound with heat-shock
`proteins, there are also reports supporting that AR largely
`resides in the nucleus [13].
`Table 1. Proposed Mechanisms of HRPC Development(ref.:
`15-18)
`
`1.
`
`•
`
`•
`
`•
`
`2.
`•
`
`•
`
`•
`
`Bypassing AR signalling pathway
`
`Ligand-independent enhancement of AR action from GFs and
`cytokines
`
`Mutations and/or deletions of AR
`
`Aberrant methylation of the AR gene promoter with consequent
`inhibition of its expression
`
`Adapting AR signalling pathway
`
`AR amplification
`
`AR mutations that change ligand specificity
`
`AR ligand-indendent activation through cross-talk with other
`signal-transducing pathways
`
`•
`
`Transcriptional co-factors participation (co-activator
`amplification or co-repressor down-regulation)
`Transcription of the AR gene is cell-type specific and in
`some tissues also age-specific. Moreover, AR messenger
`RNA (mRNA) levels are regulated by androgen and other
`steroid hormones. It is noteworthy that, except for the
`spleen, there are no other tissues that do not express AR
`[14]. Therefore, AR expression control
`through post-
`translational modifications (e.g. phosphorylations), presence
`of specific transcriptional co-factors, genetic (e.g. mutations)
`and epigenetic (e.g. methylation, acetylation) events is of
`paramount importance for tissue-selectivity determination.
`Over 80% of patients with advanced prostate cancer will
`show some kind of response to androgen blockade [15].
`Unfortunately, there are not currently available predictive
`factors to identify these patients, as well as the duration of
`this response. However, it seems that clinical responses are
`not correlated with the levels of AR in cancer tissue.
`Consistent with this, it has been demonstrated that AR
`expression is sustained even with androgen blockade [16].
`With few exceptions, the AR gene is normally expressed
`in prostate cancer. However, after hormone treatment is
`administered, significant changes are noted through which
`prostate cancer is converted from hormone-sensitive to
`HRPC [17]. This crucial point of prostate cancer natural
`history
`is still not well elucidated, although various
`molecular mechanisms have been suggested (Table 1). All
`these mechanisms finally result in the growth of prostate
`cancer cells in a low-androgen environment and enhanced
`AR activity with a broad list of ligands [18].
`
`Papavassiliou et al.
`
`3. NOVEL THERAPEUTIC STRATEGIES FOR
`HRPC
`
`The current available treatment options for HRPC are
`supportive
`care,
`salvage
`endocrine manipulations,
`radiotherapy, radioactive isotopes, biphosphonates and
`chemotherapy [19]. As all these alternatives have not offered
`a significant improvement in terms of survival, new
`strategies are being developed and evaluated (Table 2).
`Table 2. Novel Therapeutic Strategies for the Treatment of
`HRPC(ref.)
`
`1.
`
`•
`
`•
`
`•
`
`2.
`
`3.
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`Immunotherapy
`
`Vaccines21
`Activated autologous dendritic cells22,23
`Monoclonal antibodies24-26
`Gene Therapy27,28
`
`Biological Agents
`Growth factors inhibitors31-46
`Signal transduction inhibitors47-89
`Apoptosis regulators10-110
`Cell-cycle regulators112-114
`Proteasome inhibitors116-120
`Neo-angiogenesis inhibitors122-153
`Anti-metastatic agents158-166
`Differentiation agents170-203
`Epigenetic therapeutics207-214
`Telomerase inactivators215-217
`
`3.1. Immunotherapy
`
`Until recently, prostate cancer was considered as a non-
`immunogenic tumour. This assumption has changed and the
`role of immunotherapy is being extensively explored. Active
`and passive immune approaches directed against prostate-
`specific antigens, oncogenic proteins, altered
`tumour
`suppressor gene products, and differentiation antigens, are
`ongoing [20]. A variety of prostate cancer cell-surface
`glycoproteins and carbohydrates serve as potential targets of
`synthetic vaccines, which are evaluated in phase I/II trials
`with encouraging, so far, results [21]. Immunomodulatory
`cytokines and dendritic cell therapy also represent attractive
`immunological strategies with encouraging results in HRPC
`[22,23]. Monoclonal antibody (MA)-based therapeutics is
`also being applied in HRPC. After the recent clinical success
`of trastuzumab, a humanised MA against HER-2/neu
`receptor in the treatment of patients with breast cancer with
`HER-2 over-expression, its use has also been suggested for
`the treatment of patients with HRPC. However, a recently
`published clinical trial revealed that immunohistochemical
`over-expression of HER-2 is present in only a small
`percentage of patients with HRPC, 6% have 2+ and only 1%
`have 3+ immunopositivity for HER-2) [24]. Therefore, further
`clinical evaluation of trastuzumab is considered rational only
`
`

`

`Novel Biological Agents for the Treatment of HRPC
`
`Current Medicinal Chemistry, 2005, Vol. 12, No. 3 279
`
`Table 3.
`
`Important Published Clinical Trials Evaluating Novel Biological Agents Alone or in Combination with other
`Therapeutic Strategies in HRPC Treatment
`
` Agent
`
`Target
`
`Primary conclusion
`
`Phase
`
`Ref
`
`1.Growth Factor Inhibitors
`
`Trastuzumab+ Docetaxel
`
`SU101
`
`ErbB-2/HER-2
`PDGFRa
`
`2. Signal Transduction Inhibitors
`EGFR-TKb
`
`Gefitinib
`
`Gefitinib
`
`EGFR-TK
`
`Gefitinib
`
`+ Mitoxantrone/Prednisone
`
`EGFR-TK
`
`Gefitinib
`
`+ Docetaxel/Estramustine
`
`EGFR-TK
`
`ISIS 5132
`
`ISIS 3521
`
`Raf-1 kinase
`PKCc
`
`3. Apoptosis Regulators
`
`HER2 overexpression in prostate cancer is infrequent.
`
`Modest activity regarding PSA and objective clinical responses
`
`as single-agent activity in heavily pretreated patients.
`
`No objective or PSA responses were reported.
`
`Initial results reporting
`progression
`
`as single-agent treatment.
`
`infrequent PSA responses or early
`
`II
`
`II
`
`II
`
`II
`
`Preliminary results show promising PSA responses with tolerable
`toxicity.
`
`I/II
`
`Preliminary results show promising PSA
`acceptable toxicity.
`
`responses with
`
`I/II
`
`No objective or PSA responses were observed.
`
`No objective or PSA responses were observed.
`
`Well-tolerated combination without additive toxicity.
`
`II
`
`II
`
`I
`
`24
`
`43
`
`50
`
`51
`
`52
`
`53
`
`68
`
`68
`
`94
`
`Genasense + Mitoxantrone
`
`bcl-2
`
`Genasense + Docetaxel
`
`Atrasentan
`
`Atrasentan
`
`bcl-2
`d
`ETA
`ETA
`
`4. Cell-cycle Regulators
`
`Flavopiridol
`
`CDKse
`
`5. Proteasome Inhibitors
`
`PS-341 + Docetaxel
`
`proteasome
`
`6. Neo-Angiogenesis Inhibitors
`
`Suramin (fixed high dose)
`+ Hydrocortisone
`
`Suramin (monthly)
`
`Suramin (three
`doses)
`
`different
`
`Thalidomide
`
`Thalidomide
`
`Thalidomide + Docetaxel
`
`Well-tolerated combination with PSA and clinical responses.
`
`Well-tolerated agent with mild vasodilatory adverse events.
`
`Favorable responses in a variety of clinical measures, including
`
`II
`
`I
`
`II
`
`time to progression.
`
`Significant toxicity without objective responses as single-agent
`treatment.
`
`II
`
`Preliminary results show encouraging efficacy and tolerable
`toxicity.
`
`I/II
`
`High, but of short duration, efficacy with acceptable
`profile.
`
`toxicity
`
`II
`
`Reported PSA and objective responses in heavily pretreated
`patients.
`
`II
`
`No dose-response
`progression-free
`
`relationship was
`
`reported
`
`regarding
`
`and overall survival, whilst toxicity was enhanced with higher
`doses. II
`
`PSA responses reported in heavily pretreated patients.
`
`PSA responses reported in heavily pretreated patients.
`
`The combination achieved better PSA responses than docetaxel
`alone.
`
`II
`
`II
`
`II
`
`95
`
`101
`
`102
`
`114
`
`120
`
`125
`
`126
`
`127
`
`129
`
`130
`
`131
`
`

`

`280 Current Medicinal Chemistry, 2005, Vol. 12, No. 3
`
` Agent
`
`Target
`
`Primary conclusion
`
`Papavassiliou et al.
`
`(Table 3). contd.....
`
`Phase
`
`Ref
`
`+
`
`+
`
`+
`
`Thalidomide
`Mitoxantrone/Prednisone
`
`Thalidomide
`Dexamethasone (p.o.)
`
`Thalidomide
`Paclitaxel/Doxorubicin
`
`Carboxyamido-triazole
`(CAT)
`
`Bevacizumab
`
`Bevacizumab
`
`The combination did not achieve response benefit but caused
`additive toxicity.
`
`Preliminary encouraging results concerning efficacy.
`
`Preliminary encouraging results concerning efficacy.
`
`No clinical activity in HRPC patients with soft tissue metastasis.
`
`VEGFRf
`
`No significant objective responses as single-agent treatment.
`
`II
`
`II
`
`I/II
`
`II
`
`II
`
`133
`
`134
`
`135
`
`140
`
`146
`
`147
`
`150
`
`+ Docetaxel/Estramustine
`
`VEGFR
`
`TNP-470
`
`7. Anti-Metastatic Agents
`
`Prinomastat
`
`MMPsg
`
`Initial results showing
`toxicity profile.
`
`remarkable efficacy with acceptable
`
`II
`
`Reversible neuropsychiatric dose-limiting side
`transient PSA increases.
`
`effects
`
`and
`
`I
`
`with subsequent decline.
`
`No differences were found in the two treatment regimens in terms
`of
`
`III
`
` 164
`
` Versus
`
`Prinomastat/Mitoxantrone/P
`rednisone
`
`8. Differentiation Agents
`
`Calcitriol + Docetaxel
`
`acid
`
`acid
`
`acid
`
`acid
`
`All-trans-retinoic
`(ATRA)
`
`All-trans-retinoic
`(ATRA)
`
`13-cis-retinoic
`(Isotretinoin)
`
`+ Androgen Blockade
`
`13-cis-retinoic
`(Isotretinoin)
`
`+
`alpha/Paclitaxel
`
`PSA responses, progression-free survival, 1-year and overall
`
` survival.
`
`Well-tolerated combination regimen with promising results
`
`regarding PSA and measurable disease responses,
`progression and survival.
`
`time
`
`to
`
`ATRA is not active against HRPC.
`
`ATRA has minimal activity against HPRC.
`
`II
`
`II
`
`II
`
`Isotretinoin does not impair PSA response or cause significant
`toxicity.
`
`II
`
`Interferon
`
`First study evaluating the efficacy and safety of this combination,
`
`I
`
`172
`
`180
`
`181
`
`182
`
`183
`
`189
`
`209
`
`Troglitazone
`
`PPAR h
`
`9. Epigenetic zherapeutics
`
`which was well tolerated with encouraging results.
`
`Preliminary encouraging results concerning PSA responses.
`
`5-aza-2'-deoxycytidine
`
`methylation
`
`Well tolerated with modest clinical activity.
`
`(azacitidine)
`PDGFR, Platelet-Derived Growth Factor Receptor; b EGFR-TK, Epidermal Growth Factor Receptor Tyrosine Kinase, c
`Abbreviations: a
`Endothelin-A receptor; e CDKs, Cyclin-Dependent Kinases; f VEGFR, Vascular Endothelial Growth Factor Receptor;
`g
`MMPs, Matrix Metalloproteinases; h PPAR , Peroxisome Proliferator-Activated Receptor .
`
`II
`
`II
`
`PKC, Protein Kinase C; d ETA,
`
`for the subgroup of HRPC patients exhibiting HER-2 over-
`expression, either as single-agent therapy or in combination
`
`regimens. Finally, several anti-prostate-specific membrane
`MAs (APSMMAs) have been developed against both intra-
`
`

`

`Novel Biological Agents for the Treatment of HRPC
`
`Current Medicinal Chemistry, 2005, Vol. 12, No. 3 281
`
`and extracellular antigenic epitopes and are under clinical
`evaluation for radioimmunotherapy of HRPC [25,26].
`
`3.2. Gene Therapy
`
`Prostate cancer gene therapy represents a promising
`distant future treatment strategy. A number of different
`approaches are being explored, such as correcting aberrant
`gene expression, exploiting apoptotic cell pathways,
`introducing toxic or lytic “suicide genes”, targeting crucial
`cell functions, enhancing host anti-tumour immunologic
`response and various combinations [27]. Preclinical, in vitro
`and in vivo, results are encouraging, although most research
`is currently focused on the development of more effective
`vector delivery and selective targeting [28].
`
`3.3. Novel Biological Agents
`
`A great number of new therapeutic strategies are under
`development (Table 2) and early clinical evaluation for the
`treatment of HRPC (Table 3), based on the rapidly
`increasing knowledge pertaining to the molecular biology of
`the prostate carcinogenesis process.
`
`4. GROWTH FACTOR INHIBITORS
`
`Growth factors are necessary for cell proliferation. Many
`human solid tumours, such as prostate cancer are associated
`with over-expression of growth factors and their receptors,
`and the hypothesis is that dysregulated stimulation of
`growth factor receptors contributes to carcinogenesis, and
`vise versa. Experimental data have shown that among the
`mechanisms associated with the development of HRPC, is
`the bypassing of
`the AR-signalling cascade
`through
`activation of growth
`factor
`receptors and enhanced
`intracellular signalling activity with subsequent increase of
`cancer cell proliferation, inhibition of apoptosis and increased
`expression of markers of drug resistance. Therefore, targeting
`of growth factor signalling represents a possibly promising
`new therapeutic approach in treating HRPC [29] (Fig. 1A).
`
`4.1. Inhibitors of Epidermal Growth Factor Receptor
`(EGFR)
`
`EGFR, ErbB1/HER1, is one of the four known members
`of the HER-family of growth factor receptors including:
`ErbB1, ErbB2/HER2, ErbB3 and ErbB4, which are
`mediators of cell growth, differentiation and survival [30].
`Enhanced expression of EGFR has been associated with
`tumour progression in various tumours, including HRPC
`[31]. EGFRs are normally expressed in normal prostatic
`tissue, while their expression increases with androgen-
`independence [32]. Anti-EGFR therapeutic approaches in
`prostate cancer include MAs directed against the extracellular
`bonding domain, small-molecule tyrosine kinase inhibitors,
`ligand conjugates,
`immuno-conjugates and antisense
`oligonucleotides [6]. Agents in clinical development include
`IMC-C225 (cetuximab), EMD 55900, ICR 62, ABX-EGF
`and others that directly block EGFR [33,34]. In vitro studies
`have suggested that cetuximab is capable of inhibiting
`tumour growth and metastasis, while paclitaxel and
`doxorubicin enhanced these results in HRPC cells [35,36].
`A novel use of anti-EGFR MAs includes their combination
`
`with MAs targeting other tumour antigens, such as HER-2.
`GW572016 (lapatinib)
`is a reversible small-molecule
`selective dual inhibitor of both EGFR and ErbB2 tyrosine
`kinases, which has recently entered clinical trials as an oral
`agent [37].
`
`4.2. Inhibitors of Platelet-Derived Growth Factor
`Receptor (PDGFR)
`
`to be potent
`The PDGF proteins are suggested
`stimulators of cell proliferation and play a major role in
`intracellular communication. Experimental data have shown
`that PDGFRs are expressed in prostatic intraepithelial
`neoplasia and carcinoma, but not
`in benign prostate
`hypertrophy or normal prostatic epithelium [38], suggesting
`that PDGF signalling might significantly contribute to the
`development of primary and metastatic prostate cancers [39].
`Imatinib mesylate (ST1571-Gleevec®) has been found to
`exert a direct inhibiting action towards the bcr-abl kinase
`activity with significant clinical effects, thus its use in
`patients with chronic myeloid leukemia and Ph+ acute
`lymphoblastic leukemia represents a valid therapeutic option
`[40]. It has also been found that imatinib mesylate is a
`potent inhibitor of PDGFR kinase and clinical trials are
`underway to evaluate its efficacy in the treatment of patients
`with HRPC [41,42].
`SU101 (leflunomide) is also a novel potent and highly
`selective inhibitor of PDGFR. After the encouraging results
`observed in phase I studies, a large scale phase II study of
`SU101 as monotherapy of HRPC patients resulted in a
`modest objective clinical benefit with the most frequent
`adverse events being nausea, anorexia and anemia [43].
`Despite these results, further studies are warranted to assess
`the efficacy of SU101 either as a single treatment agent or in
`combination regimens for the treatment of HRPC.
`
`4.3. Inhibitors of Insulin-like Growth Factor Receptor
`(IGFR)
`
`IGFR is suggested to be involved in tumour cell
`proliferation, invasion and survival. Several studies have
`indicated that IGF axis contributes to prostate cancer
`progression [44]. Recently, it was suggested that a direct
`correlation exists between IGFR inhibition and down-
`regulation of zinc-dependent matrix metalloproteinase-2
`(MMP-2), as well as with increased rate of apoptosis in
`androgen-independent cancer cells [45]. However, differential
`expression of certain IGF family members has been recently
`reported in various histological entities during prostatic
`carcinogenesis [46]. Therefore, thorough understanding of the
`role of these growth factors and their associated ligands and
`receptors will elucidate their potential therapeutic application
`in HRPC.
`
`5. SIGNAL TRANSDUCTION INHIBITORS
`
`Cancer cells receive external signals through surface
`receptors that stimulate their growth and proliferation. The
`transduction of the membrane-bound receptor activation
`signal to the nucleus is achieved and enhanced through
`various intracellular biochemical reactions. All these signal
`transduction molecular pathways are often dysregulated
`
`

`

`282 Current Medicinal Chemistry, 2005, Vol. 12, No. 3
`
`Papavassiliou et al.
`
`during carcinogenesis. The deeper understanding of these
`molecules and their downstream and cross-talk relationships
`has generated intense research efforts in designing specific
`inhibitors of key proteins that are gradually entering into the
`clinical evaluation phase in the treatment of patients with
`solid tumours, including HRPC (Fig. 1B).
`
`5.1. Tyrosine Kinase Inhibitors
`
`Inhibition of growth factor receptor kinase-dependent
`signalling pathways is one of the most promising novel
`treatment strategies for prostate cancer treatment [47]. EGFR-
`tyrosine kinase (EGFR-TK) activity leads to activation of
`downstream pathways such as ras/MAP kinase and STATs
`(signal
`transducers and activators of
`transcription)
`transcription factors [30]. These signal transduction events
`are critical for the growth of many human tumours. EGFR
`has been found to be over-expressed in a wide range of
`human cancers, including prostate cancer [31], while this
`over-expression seems to be associated with poor outcome
`[32]. Several small-molecule inhibitors of EGFR-TK have
`been developed, such as ZD1839 (Iressa®), OSI-774
`(Tarceva®), PD182905, PKI-166 and CI-1033 [48]. Iressa,
`given either intermittently or continuously, has resulted in
`remarkable efficacy with low-toxicity profile in patients with
`prostate cancer in recent phase I/II studies [49-51]. In
`addition to their single-agent activity, EGFR-TK inhibitors
`have also shown synergy with chemotherapy and radiation
`therapy. Phase
`I/II clinical
`trial evaluations of
`the
`combination of ZD1839 with mitoxantrone/prednisone [52]
`and docetaxel/estramustine [53], resulted in 23% and 33%
`decreases in PSA responses respectively, with no additive
`toxicity. An oral pan-ErbB TK irreversible inhibitor, CI-
`1033 [54], and PKI-166, a dual ErbB1/ErbB2 TK inhibitor
`[55], have recently reported to have in vitro antitumour
`activity against prostate cancer cells either alone or in
`combination with radiation therapy and are now entering the
`clinical testing phase. Oncogenic TKs seem to represent an
`attractive anti-tumour target. However, combination of TK
`small-molecule inhibitors and conventional chemotherapy
`and/or radiation therapy might result in better clinical
`results.
`
`5.2. Farnesyl-Protein Transferase (FPTase) Inhibitors
`
`Ras family proteins regulate important growth factor
`receptor-induced signalling pathways contributing to cellular
`differentiation and proliferation. Key to the functionality of
`Ras proteins is the post-translational farnesylation of the
`amino-terminus of Ras by a cytosolic enzyme, the so-called
`farnesyl-protein transferase (FPTase) [56]. Many solid
`tumours, such as prostate cancer, have been reported to
`exhibit Ras dysregulation [57]. Although agents that either
`down-regulate Ras expression or reverse Ras activation have
`not been developed yet, several FPTase inhibitors (FPTIs)
`are currently under clinical evaluation [58].
`A number of FPTIs are undergoing phase I/II trials both
`as monotherapy and in combination with chemotherapeutic
`agents in HRPC [59]. Combined efficacy was demonstrated
`with FPTIs SCH66336 and SCH58500
`in androgen-
`insensitive DU145 prostate cancer cells [60]. Tipifarnib
`(R115777) has been broadly studied in a wide variety of
`
`tumour types with myelosuppresion being the major toxicity
`[61]. Moreover, FPTIs can antagonise the growth enhancing
`properties of Ras-related small GTP-bonding proteins such
`as Rho and Rac. Notably, the Rho kinase inhibitor Y-27632
`has shown to inhibit tumour growth and angiogenesis in
`androgen-insensitive prostate cancer cells [62]. All these
`findings support the further clinical evaluation of FPTIs
`alone or in combination treatments in HRPC.
`
`5.3. Raf Kinase Inhibitors
`
`Raf-1 is a downstream kinase of the activated Ras
`signalling pathway [63]. The first orally active inhibitor of
`Raf kinase, BAY 43-9006 (sorafenib), has been assessed in
`phase I studies in patients with various solid tumours,
`among them prostate cancer [64-67]. Antisense technology
`has also been used for the development of Raf kinase
`selective inhibitors. The Raf kinase inhibitor ISIS 5132 has
`been evaluated in phase I/II trials in HRPC patients and it
`was found to lack significant effects on the PSA response and
`to have a non-tolerable toxicity profile [68].
`
`5.4. Mitogen-Activated Protein Kinase
`Inhibitors
`
`(MAPK)
`
`the
`MAPK pathways have been associated with
`progression of prostate cancer to its androgen-independent
`stage [69]. At least four distinct groups of MAPKs have been
`identified: (a) ERKs, extracellular signal-related kinases, 1/2;
`(b) JNK, c-Jun amino-terminal kinases, 1/2/3; (c) p38,
`-
`proteins; and (d) ERK5 [8]. It has been suggested that in
`HRPC the ERK cascade is relatively activated whereas the
`stress-activated protein kinase
`(SAPK) pathway
`is
`suppressed [70]. Components of MAPK pathways represent
`candidate targets for therapeutic intervention [71]. Differential
`regulation of the interaction between MAPKs can be
`accomplished with selective inhibition, i.e. by phosphatases
`such as Cdc25A phosphatase [72]. The MAPK inhibitors
`PD098059 and PD184352 are among novel MAPK small-
`molecule inhibitors that have been developed and evaluated
`in preclinical prostate tumour models [73,74]. Recently, it
`was also demonstrated that angiotensin II induces the
`phosphorylation of MAPK in androgen-independent prostate
`cancer cells. Oral administration of angiotensin II receptor
`blocker was found to inhibit the growth of prostate cancer
`xenografts in a dose-dependent manner [75]. Therefore, these
`agents are currently being evaluated as a therapeutic option
`for HRPC.
`
`5.5. Mammalian Target of Rapamycin
`Inhibitors
`
`(mTOR)
`
`The tumour suppressor gene PTEN (phosphatase and
`tensin homologue) on chromosome 10q23 is the most
`frequently mutated gene in prostate cancer [76]. Although its
`expression remains in normal prostate. Prostatic carcinomas
`usually exhibit down-regulation of this protein supporting a
`possible key role for gradual PTEN functional loss in the
`progression of prostate cancer to its hormone-independent
`state [77]. PTEN functions as a negative modulator of the
`phosphoinositide-3 kinase (PI3K)/Akt signal transduction
`pathway [78]. Akt protein is a tyrosine/threonine kinase with
`various modulatory effects regarding cellular proliferation,
`
`

`

`Novel Biological Agents for the Treatment of HRPC
`
`Current Medicinal Chemistry, 2005, Vol. 12, No. 3 283
`
`translational control, while a possible
`apoptosis and
`relationship of activated Akt and AR expression has also
`been postulated [79]. In addition, mammalian Target of
`Rapamycin (mTOR) is a downstream kinase implicated in
`Akt-mediated translational control [80]. It has also been
`reported that mTOR inhibition results in cyclin-dependent
`kinases (CDKs) inhibition, Rb phosphorylation inhibition
`and enhancement of cyclin D1 degradation, culminating in
`cell-cycle arrest in G1 phase [81]. Therefore, blockage of
`mTOR function should lead to inhibition of PI3K/Akt-
`mediated proliferation signals and cell arrest [82].
`Rapamycin, which is a macrolide fungicide, and its
`analogs, CCI-779, RAD001 and AP23573 have been found
`to share mTOR inhibitory properties, thus suppressing its
`carcinogenic potential [83]. The CCI-779 analogue has
`shown a significant anti-proliferative effect and favourable
`toxicology profile in phase I/II trials in various human
`cancers, including HRPC [84,85]. Major toxicities observed
`with
`CCI-779
`included
`anemia,
`hyperglycemia,
`hypertriglyceridemia,
`hypophosphatemia,
`stomatitis,
`mucositis and bowel perforation. Several studies evaluating
`the combination of mTOR inhibitors with chemotherapeutic
`agents in HRPC are being developed.
`
`5.6. Protein Kinase C (PKC) Inhibitors
`
`PKC is a negative growth regulator of human prostate
`cancer cells due to its involvement in triggering apoptosis
`[86]. Antisense oligonucleotides targeting PKC have shown
`promising
`results, either as a monotherapy or
`in
`combination with chemotherapeutics
`in various solid
`tumours, such as lung and prostate cancers [87]. Preclinical
`
`evidence suggested that ISIS 3521 causes specific inhibition
`of PKC mRNA with accompanying anti-tumour activity.
`Phase I/II clinical trials have been completed or are ongoing,
`evaluating this agent in HRPC [68].
`
`5.7. Ansamycins
`
`The benzoquinoid ansamycin antibiotics are derived from
`Streptomyces hygroscopicus and include geldanamycin and
`herbimycin. Ansamycins appear to have an antitumour effect
`based on their action against chaperone proteins (i.e.,
`Hsp90), which are responsible for maintaining the active
`conformation of selected protein kinases, steroid receptors,
`Raf-1 kinase, cyclin D1 and EGFR [88]. One agent can
`thereby target multiple protein kinases, including the HER-2
`axis, as well as the AR, a particularly promising strategy for
`the treatment of prostate cancer [89]. Phase I clinical trials in
`patients with HRPC are underway.
`
`6. APOPTOSIS REGULATORS
`
`Apoptosis, the programmed cell death mediated by
`proteases called caspases, is essential for normal tissue
`functions. Impaired apoptosis is a central step during
`prostate cancer natural history [8]. Anti-apoptotic bcl-2
`family members and pro-apoptotic proteins interplay control
`the release of cytochrome c from
`the mitochondrial
`membrane, activation of the caspase cascade and apoptosis
`execution. Conventional cytotoxic and radiation therapy
`indirectly induces apoptosis, but outcomes that are more
`effective should be achieved by direct activation of the
`apoptotic machinery. Thus, an alternative way to modulate
`
`CH3
`
`O
`
`H3C
`
`O
`
`O
`
`O
`
`N
`
`CH
`
`l
`
`OC
`
`N
`
`HN
`
`O
`
`CIH
`
`OSI-779
`
`N
`
`N
`
`HN
`
`GW572016
`
`F
`
`HN
`
`SO
`
`O
`
`H3C
`
`N
`
`F
`
`F
`
`F
`
`NH
`
`O
`
`CH3
`
`SU 101
`
`N
`
`O
`
`F
`
`Cl
`
`CH3
`
`NH
`
`N
`
`N
`
`N
`
`N
`
`HN
`
`NH
`
`O
`
`CH3
`
`O O
`
`ZD1839
`
`A
`
`N
`
`O
`
`H3C
`
`N
`
`N
`
`HO
`
`O
`S
`
`O
`
`CH3
`
`ST1571
`
`

`

`284 Current Medicinal Chemistry, 2005, Vol. 12, No. 3
`
`Papavassiliou et al.
`
`CH3
`
`O
`
`H3 C
`
`O
`
`O
`
`Cl
`
`N
`
`H2 N
`
`H2C
`
`N
`
`N
`
`N
`
`HN
`
`H3C
`
`HN
`
`O
`
`O
`
`F
`
`Cl
`
`(Fig. 1). contd.....
`B
`
`CH3
`O
`
`O
`
`N
`
`ZD1839
`
`NN
`
`H
`
`O
`
`F
`
`Cl
`
`N
`
`CI-1033
`
`N
`
`O
`
`N
`
`CH
`
`CIH
`CIH
`
`CH3
`
`NH2
`
`N
`
`O
`
`NH
`
`Y-27632
`
`N
`
`HN
`
`O
`
`CIH
`
`OSI-779
`
`CH3
`
`N
`
`O
`
`R115777
`
`Cl
`
`O CH2
`O
`
`O
`
`OH
`
`CH2
`OH
`
`CH3
`
`O
`
`HN
`
`N
`
`HN
`
`HN
`
`O C
`
`H2
`
`OH
`
`O
`HO
`
`H2C
`
`H2C
`
`N
`
`O
`
`O
`
`O
`
`O
`
`O
`
`O
`
`H2 C
`
`H2C
`H2C
`
`CH2
`
`CCI-779
`
`CH2
`
`CH3
`
`CH3
`
`H3
`
`OC
`
`CH3
`
`O
`
`NH2
`
`O
`
`O C
`
`H3
`
`O CH3
`
`O CH3
`
`CH3
`
`O
`
`NH
`
`O
`
`O
`
`Herbimycin
`
`O
`
`Cl
`
`F
`
`F
`
`F
`
`O
`
`CH3
`
`O
`
`BAY 43-9006
`
`CH3
`
`O
`
`NH
`
`O
`
`O
`
`O
`
`H3C
`
`H3C
`
`OH
`
`H3C
`
`O
`
`O
`
`CH3
`
`CH3
`
`CH3
`
`Geldanamycin
`
`NH2
`
`O
`
`O
`
`

`

`Novel Biological Agents for the Treatment of HRPC
`
`Current Medicinal Chemistry, 2005, Vol. 12, No. 3 285
`
`(Fig. 1).contd.....
`
`CH3
`
`HN
`
`O
`
`CH3
`
`HO
`
`OH
`
`NH
`
`O
`
`PS-341
`
`NN
`
`OH
`
`CH3
`
`H
`
`H
`
`H
`
`2-methoxyes tradiol
`
`H3
`
`NC
`
`O
`
`H3C
`
`HO