RAPAMYCIN: CLINICAL RESULTS AND FUTURE OPPORTUNITIES1

INTRODUCTION Sirolimus (rapamycin; RAPA) is a macrocyclic lactone with a novel mechanism of immunosuppressive action (1). During the past 7 years, the drug has undergone clinical trials progressing from Phase I safety, tolerability, and pharmacokinetic investigation to Phase II dose-finding studies and limited-sized, multicenter evaluations of drug combination regimens. The completion of Phase III large randomized national and international trials led to approval of the drug to achieve augmented acute rejection prophylaxis in combination with cyclosporine (CsA) and steroids by the Food and Drug Administration of the United States in September 1999. In November 2000, the drug was approved by the European Agency as an alternate to calcineurin antagonists for long-term maintenance therapy. This overview seeks to familiarize the reader with the clinical information that provided the bases for drug approval and with the single-center reports that document alternate approaches to optimize the outcomes of treatment with this immunosuppressive agent. I. Update on Preclinical Findings RAPA, via its c-7 methoxy group (2), cross-links (3) the immunophilin FK binding protein (FKBP) 12, a peptide-prolyl isomerase that acts as a folding catalyst, to the multifunctional serine-threonine kinase, the mammalian target of rapamycin (mTOR) (4). Blockade of mTOR dampens lymphocyte responses to costimulatory signal 2 during the G0 to G1 transition and to cytokine signal 3 during the G1 build-up. By blocking the costimulation signals, RAPA prevents activation of the inhibitory factor kappa kinase necessary for generation of the c-Rel transcription factors of the NF-κB complex (5), and possibly also modulates protein kinase C activity (6). During the later G1 phase, by blocking signal 3, RAPA inhibits four cytokine-driven signaling pathways: a) p27kip1 degradation (7,8) leading to cyclin activation (9,10); b) p70S6 kinase stimulation, a step necessary for the synthesis of endosomal structural proteins (11–13); c) elongation factor 4A release from its association with PHAS-I, thereby facilitating ribosomal protein synthesis (14–16); and d) transcriptional up-regulation of the anti-apoptotic proteins bcl (17,18) and p21Ras (19) (Fig. 1). Both the therapeutic and the toxic effects of RAPA are related to the same cellular actions. The drug’s unique effects are complementary to calcineurin antagonists (CNAs) (20) and to interleukin-2 receptor monoclonal antibodies (anti–IL-2R mAbs); a relation that has been called the “cytokine paradigm”(21) (Fig. 2). Figure 1: Sites of enzyme action of mammalian target of rapamycin (mTOR). a) Activation of c-Rel factors downstream from reception of the costimulatory CD28 signal. b) Phosphorylation of p70S6 kinase preceding endosomal structural protein synthesis. c) Release of e-IH-4E from its association with PHAS-I, leading to the 4E activity necessary for elongation of the polypeptide chains on ribosomes. d) Dissociation of p27kip1 from cyclin C kinase, promoting cell division and up-regulated expression of bcl, an anti-apoptotic factor.Figure 2: The cytokine paradigm includes calcineurin antagonists (cyclosporine [CsA] or tacrolimus [TRL]) to block the antigen-driven signal 1 and RAPA to inhibit costimulatory signal 2, thereby mitigating transcriptional activation of cytokine synthesis during progression from G0 to G1. mAbs to IL-2R block the capacity of IL-2 to trigger signal 3 transduction events, which are inhibited by RAPA, preventing G1 progression (modified from reference 21).The preclinical development of RAPA has been extensively reviewed (22). After documentation of its immunosuppressive activity in animal models by the groups of Calne (23) and later by Morris et al. (24), RAPA was demonstrated to display a high degree of synergy with CsA both in vitro (25) and in vivo (26) by the rigorous median effect analysis. These findings provided the foundation for the drug’s initial clinical development in renal transplantation, anticipating that synergy with lower doses of CsA would not only more effectively prevent rejection, but also minimize CNA-induced toxicity. Two observations suggest other unique properties of RAPA that may be exploited in future controlled clinical trials. First, high doses of RAPA block the proliferative responses to cytokines by vascular and smooth muscle cells after mechanical injury, such as balloon angioplasty, or allorejection (reviewed in 27). In a nonhuman primate model, supratherapeutic concentrations of RAPA stabilized, and possibly reversed, the intimal vascular lesion caused by the progression of immune injury in aortic allografts (28). Studies are underway to assess the potential contributions of adjunctive agents to potentiate this effect at therapeutic levels of RAPA. Second, by virtue of its inhibition of bcl-2, RAPA may produce a tolerogenic pro-apoptotic effect, in apparent contradistinction to high doses of CNAs. RAPA treatment concomitant with mAb blockade of the costimulatory signal by anti-CD154 in mice induces tolerance (29,30), and the combination of RAPA and anti-B7 in nonhuman primates seems to facilitate graft survival (31). Thus, RAPA may mitigate the vasculopathic response to immune or mechanical injury and facilitate tolerance induction. A compelling aspect of RAPA therapy is the absence of the vasomotor renal side effects exhibited by the CNAs CsA and tacrolimus (TRL). Treatment with RAPA preserves glomerular filtration rates (GFR) and renal blood flow in normal (32), salt-depleted (33), and spontaneously hypertensive (34) rats, as well as in micropuncture preparations (35). Although initial studies in salt-depleted rats suggested that high doses of RAPA potentiate CsA-induced nephrotoxicity (36), recent experiments demonstrate that these adverse effects are caused by pharmacokinetic (PK) interactions that elevate renal tissue CsA concentrations disproportionately to whole blood drug levels (37). Indeed, a median effect analysis based upon renal tissue CsA concentrations suggests that RAPA displays a protective effect, which has been postulated to be related to inhibition of the intrarenal angiotensin II cascade (37). However, RAPA does produce a dose-dependent tubular toxicity in rats, which seems to be caused by delayed recovery of tubular epithelial function after injury (36). II. Clinical Pharmacology An understanding of the PK behavior of immunosuppressants is critical to guide the selection of doses and administration schedules, to predict food and drug interactions, and to assess the impact of ethnicity, age, gender, and organ function on drug exposure. The RAPA data presented in Table 1 were derived from complete concentration-time profiles in 690 subjects, and trough (Cminss) measurements from nearly 1000 patients in 40 clinical studies. The subjects included healthy volunteers, stable and de novo renal transplant recipients, children and adults on dialysis therapy, and patients with hepatic impairment or psoriasis. Table 1: Steady state pharmacokinetic parameters of RAPA in various patient populationsRAPA has been detected in whole blood samples by two high performance liquid chromatography (HPLC) methods specific for parent compound; ultraviolet wavelength (UV) (38,39) and mass spectroscopy (MS) (40). A third automated immunoassay (IMx, Abbott, N. Chicago, IL) is less selective for parent compound because it displays a 42.5% cross-reactivity with metabolites (41). Because the parent compound, not metabolite, concentrations determine biologic activity (42), HPLC/UV and HPLC/MS are the reference measurement methods used at present. RAPA systemic bioavailability (F) is approximately 14%, and the drug shows dose proportionality (43) with a maximal concentration at about 1 hr. RAPA is manufactured as an oral solution and a tablet, which are bioequivalent (44,45). RAPA is widely distributed in tissues (19 L/kg) (46) and more extensively partitions into blood cells (B) compared with plasma (P), with B/P ratios ranging from 36 in renal transplant recipients to 79 in healthy volunteers. The results of in vitro experiments using human liver microsomes suggest that cytochrome P450 3A4 is the major biotransformation system (47), generating inactive hydroxy, di-hydroxy, hydroxy-demethyl, didemethyl, 7-0 demethyl, and 41-0 demethyl metabolites (48). More than 90% of drug-associated radioactivity has been recovered in feces. Urine represents a minor route of elimination (2.2%). The average elimination half-life (t1/2) of 60 hr, albeit dose-independent, shows the greatest interpatient variation, particularly among individuals with hepatic impairment (110 hr) or in the pediatric age group (as low as 10 hr), but not among subjects of African-American versus Caucasian ethnicity. Adult stable and de novo renal transplant recipients display 38% intersubject and 45% intrasubject coefficients of variation in steady-state oral clearance (unpublished data on file, Wyeth-Ayerst Research). Because of this variability, therapeutic drug monitoring is recommended. Cminss determinations provide an adequate index of RAPA for clinical a with (Fig. Figure trough (Cminss) with the concentration-time in de novo renal transplant The shows the which with is a of and Cminss The the from reference with the of but the of RAPA administration with or interactions with other that as for RAPA is by and and by and 1). RAPA to the of patients with a in concentrations which were not patients on RAPA versus CsA therapy concomitant therapy with RAPA in than CsA and the pharmacokinetic the side effects that are by the two greatest is the RAPA and RAPA concentrations are by concomitant versus administration of the of CsA (Fig. but not by concomitant administration of a of in the adequate drug is critical for RAPA CsA approximately because of for by 3A4 and possibly drug by Figure on RAPA of CsA administration concomitant with A or after CsA using a in The the two was by from reference with Clinical Phase I and II studies. The clinical a randomized to the of RAPA to versus to the of renal transplant patients 2). A dose-dependent in to a was by and were in blood or liver function Table 2: Phase and III clinical trials of RAPA in renal the Phase 40 recipients of renal were de novo with doses of RAPA and to a of to CsA and doses of An African-American of a was in the dose group the only acute rejection Because of the by the other patients acute rejection two of 10 recipients were with 7 RAPA and to CsA by at 1 or 1 after The acute rejection in group an acute rejection of among the group in to in a that of may be in A multicenter Phase demonstrated the administration of at doses drug the of RAPA to the rejection rates among recipients of renal as low as by patients CsA with RAPA Two Phase II studies the of RAPA as therapy. Although or acute rejection rates to or renal transplant function at and was among the RAPA groups in both studies. the initial immunosuppressive of a combination the of long-term two large trials were in which CsA was at 3 from the of patients delayed graft function an acute rejection as after transplantation, the renal function among patients from CsA was was than on the of acute rejection was but not and to augmented therapy In these studies suggest a of for RAPA a combination to CsA using a RAPA or a of therapy by long-term Phase studies are underway to assess the and of these alternate Phase III trials. studies using randomized and were to document the therapeutic of RAPA. transplant randomized patients after the renal graft initial The was based upon because of the impact of the African-American factor on outcomes in and randomized patients transplantation, patients based upon versus into a Both two patients at dose of RAPA or versus patient for the or in combination with a of CsA and a for of prophylaxis was for and for only in of to were to therapy was Both trials demonstrated that of RAPA to the at and of the clinical of a acute rejection graft to or (Fig. The of was the of rejection which was also at both RAPA groups a in the of and of rejection, as well as in the of preparations to rejection The and of graft and of were among the groups in the multicenter Phase III at and has been in the renal the of patient in the Phase III trials were and or The at the among the group in the was among the Because the trials that led to approval a rigorous the effects of a in not be CsA to be in to achieve rejection prophylaxis in the However, a CsA and RAPA was by a analysis of whole blood concentrations in samples at 2, 3, and and at 2, 3, and that RAPA Cminss concentrations of approximately 10 in combination with low CsA approximately rejection rates to during the of such In the of RAPA, CsA approximately in immunosuppressive (Fig. of the median effect that the combination 90% of patients of acute rejection at CsA lower than for the or and at RAPA concentrations lower than with These findings therapeutic synergy and the results of the preclinical studies. Figure RAPA the of and of acute rejection at in both the and the trials. The shows the of acute rejection to graft and among patients in treatment The of patients randomized is by at the bases of the by are the for the rates and the for the acute rejection patient and acute rejection rates using an analysis of graft and in the multicenter Phase III of the of an acute rejection during the as a function of the concentrations of CsA and RAPA. A was used to the of an acute rejection as a function of RAPA Cminss by the in the and the CsA Cminss with a selective mAb in the selective The at shows the impact on the of acute rejection of in CsA Cminss among patients in the and groups of the Phase III trials. The shows the impact on the of acute rejection of RAPA Cminss among patients in the RAPA 2 and RAPA groups at CsA Cminss for and to more than patients at dose of RAPA. In the Phase III trials patients RAPA for for and for from therapy 1 an of tolerability, at in the 2 RAPA in the in the RAPA and in the rates for were in the and lower in the 2 RAPA and in the RAPA the for adverse was among the RAPA and about for of the other Thus, the dose was more but less well for the in the RAPA group of to the Phase III data in among groups at In the rates of and were in dose The of in patients in prophylaxis been Table and among patients in both trials the the of in the RAPA group was a than in other groups but of the in studies of other immunosuppressive agents the of of patients RAPA and to treatment with other immunosuppressants such as or a recent suggests that a RAPA has an inhibitory effect on In the of was among patients in the and the RAPA groups in the and in the than the RAPA groups for the multicenter Because of its effects in vitro on a of cell RAPA has been to be for patients as treatment for liver However, are necessary that the effects of RAPA be to the or the progression of suggested by findings in preclinical RAPA does not the factors with transplantation, renal function was in patients with and with 3, or of RAPA for These groups in (Fig. the Phase and studies at and among patients compared with patients (Fig. However, renal tubular been among patients with a and Figure of RAPA on renal function in Phase and III clinical trials. The at the of the are the and at the are the of patients in the a) levels after 3 treatment of patients with 3, or RAPA Phase II data on file, Research). b) of among patients with or versus or by analysis of c) of of patients with in combination with RAPA 2 or RAPA The the RAPA and the other groups as by analysis of was at and d) among patients as a function of CsA with were at 3 or on CsA therapy by analysis of and the levels by patients in both Phase III studies were than of the or (Fig. Because RAPA seems to not glomerular toxicity by it seems that these CsA than RAPA toxicity and a clinical of the PK in the (37). Indeed, recent studies suggest that function be albeit by or elimination of CsA using RAPA as therapy In a Phase II of renal transplant recipients and a Phase III of subjects were randomized at 3 to CsA from a renal function at and and in and blood compared with patients on maintenance doses of CsA (Fig. These results are with the that CsA an in the effects of the Thus, RAPA a for therapy to long-term patient to CNAs with of renal the for approval by the European RAPA has been to elevate blood both in the absence and in the of CsA are also to produce in Although RAPA dampens activity in cell a effect was not in In RAPA the of in it in and studies suggest that RAPA treatment and caused by delayed clearance of The trials during the 2 after transplantation, of among both RAPA groups and the and groups an in possibly related to CsA to in and to an in However, of the RAPA a dose-dependent in the of during the after transplantation, patients more adjunctive treatment with therapy in and versus and and versus for RAPA and RAPA 2 versus groups in the and the were not for the and versus but to be for the and versus therapy for was less and versus and and versus Both and were well and in side effects of these were in patients on RAPA versus therapy. and that the of 2 years, the treatment groups was the of a of of patients (Fig. or (Fig. therapy, the of patients and by and particularly by Figure levels among patients in the Phase III multicenter a) b) The and shows the as the the median as the the and as the of the the and 90% as the on the and patients as for Phase II trials RAPA CsA and the of and were among versus patients at 2 years, therapy was in versus of Although the for RAPA and were not with adverse clinical events, such as or or with patient the to the impact of RAPA treatment in the of patients in the the caused by the in among patients with to be 1000 patients However, because it is to the long-term the has a The effects of RAPA on cell progression by cytokines or vascular factors are but less its therapeutic effect on The which to be for than has been to be and and in a single-center analysis of patients and in an analysis of the Phase III studies. adverse Two of other adverse been in clinical trials. is the of the well adverse to CsA and possibly An of such effects was not among the of patients with that are caused by CsA toxicity than by a effect of RAPA. A of adverse and possibly and of CsA in trials among patients and transplant recipients on RAPA therapy. A recent suggests that RAPA was with a that seems to be both and This may be to with other immunosuppressants such as or Although patients of RAPA has been to produce less in than both in the Phase II studies and in a preclinical patients an of which has been to in to were in the of patients of among in the Phase III trials. Table of adverse among patients treatment with RAPA 2 or or Drug The clinical results of RAPA therapy in be with the of therapeutic drug concentration particularly the to minimize the to or concomitant treatment with a The of RAPA, which was in both and recipients, was of blood CsA to CsA Cminss RAPA patients a lower acute rejection than of the A single-center analysis of the therapeutic 90% rejection prophylaxis versus toxic concentrations RAPA Cminss concentration ratios of versus the patients RAPA Cminss an for and the of initial RAPA target concentrations of de novo combination therapy with a synthesis and steroids was with a of acute rejection in Phase II trials. Thus, at during the initial it seems that more adjunctive than the synthesis be to acute rejection rates of less than studies are the in CsA to the in CsA dose caused by PK is to mitigate or after a of combination therapy be necessary to optimize long-term renal RAPA for therapy. An of are from by such as of age, a of or These display delayed graft function a that may be by the nephrotoxicity of CNAs. Because immunosuppressive therapy with preparations only a for recovery of renal and because to large of CNAs may to the are RAPA is to be in this to provide a of of The acute rejection among patients using a was in to with a combination by CsA were which at approximately 1 Because of a of acute rejection among African-American or the The rejection has been to less than with the combination of initial RAPA Cminss and adjunctive preparations than Thus, RAPA an alternate to facilitate recovery from the of rejection display clinical and vascular cellular of rejection treatment with a to of preparations are the to a immunosuppressive agent. initial the of RAPA to a in a because of II vascular 2 of the of RAPA, of clinical rejection, leading to and to a normal and of patients a of rejection which was than the in a with to a However, the RAPA of that in this it may be to RAPA Cminss and to CsA by recent reports the of RAPA in of rejection among liver and were from the in of recipients of renal 1 and patients after A multicenter from of of recipients at 3 recent with renal transplant recipients from steroids at various a of with a of because of of rejection was necessary in only During the to were of patient patient and transplant because of Thus, it seems that patients de novo with after 1 or 3 acute rejection and with only a of In Phase studies are to assess the versus the of RAPA for treatment in transplant patients on maintenance therapy. In liver transplantation, for et al. the of maintenance treatment with RAPA Phase studies are also to drug with RAPA as a for which has been in but may also be in such as human in which steroids may a with other immunosuppressive Although the effects of CsA with RAPA been the has only been in a to This is because the initial in vitro suggested that the agents an and the in vivo studies in animal models to the effects of PK interactions these two related A has not been in a controlled and concentration for therapy of RAPA with after liver are not based on a of patients at suggests that Cminss of are with a low of acute is also not the synthesis or to a particularly because the combination seems to compared with a This effect is possibly caused by plasma concentrations a to that with its structural The may to the immunosuppressive in an animal are the initial clinical trials of RAPA in combination with agents signal 2, such as anti-B7 in of the of tolerance in nonhuman primate models The action of RAPA to inhibit signal transduction by cytokines and factors in a of cells a unique to both the immune response and inhibit The of RAPA low tissue clearance and via cytochrome P450 3A4 The among which are with drug monitoring for the of RAPA as well as its with RAPA has been extensively in clinical Phase and III trials both with and concomitant CsA therapy. The combination a in the and of acute rejection A median effect analysis of drug blood concentrations by patients in the two Phase III trials that the combination displays Although patients in the not display an of or a of of adverse such as renal These to be by or elimination of However, other effects to be such as and and these to be in patients by of RAPA exposure. RAPA a and unique for immunosuppressive in organ The the of and for in the data presented in this for and for of the

Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.