Current Bioactive Compounds - Volume 7, Issue 1, 2011

PROLOGUE Coupling of low-molecular weight anticancer drugs to synthetic polymers or serum proteins through a cleavable linker has been an effective method for improving their therapeutic index through active and targeting approaches and convincing proofof- concepts have been obtained preclinically for a substantial number of prodrug candidates [1]. Drug conjugates with synthetic polymers or serum proteins that have undergone clinical assessment have used clinically established agents such as doxorubicin, camptothecin, paclitaxel, methotrexate and platinum(II) complexes. In 2007, we reviewed those carrier-linked prodrugs that had or were undergoing clinical trials [2]. Meanwhile several clinical trials with carrier-linked prodrugs have been discontinued but other candidates have moved into to the clinical setting or advanced to later stages of clinical trials. Thus, I believe that an update of ongoing clinical trials with macromolecular prodrugs in cancer therapy would prove to be a timely resource for interested parties in this particular field of drug delivery. Seven macromolecular prodrugs are currently in clinical development (see Table 1) that are based on passive tumor targeting. Clinical phase I or phase II studies with N-(2-hydroxypropyl)acrylamide (HPMA) copolymer conjugates with doxorubicin (PK1 and PK2) as well as with camptothecin (MAG-CPT, PNU166148) and with paclitaxel (PNU166945) have not been continued (for a review of these clinical trials see [2]). OPAXIO™ (paclitaxel poliglumex, CT-2103; formerly known as XYOTAX™) is a biodegradable polyglutamate conjugate of paclitaxel that has shown improved efficacy in phase III studies in non-small cell lung cancer patients over paclitaxel, but only in a subpopulation of women with low estrogen levels and phase III studies will be repeated in defined patients with non small lung cancer, but their status will not be reviewed in this edition (see http://www.celltherapeutics.com/opaxio) and Singer et al.: Biological and clinical characterization of paclitaxel poliglumex (PPX, CT-2103), a macromolecular polymer-drug conjugate. Int. J. Nanomedicine, 2006, 1, 375-383. Review). A DACH platinum HPMA complex (AP5436) is the only drug conjugate with a HPMA copolymer that is still being pursued in phase II clinical trials (see article by David P Nowotnik). The clinical development of Pegamotecan, a 4-arm PEG conjugate with camptothecin (CPT) developed by Enzon, Inc. has been replaced by EZN-2208, an analogously constructed PEG conjugate with an active metabolite of camptothecin SN38 (see article by Hong Zhao). The clinical development of two other camptothecin polymer conjugates, DE-310, a carboxymethyldextran conjugate with a CPT analogue (exatecan), and PG-Gly-CPT (CT-2106), in which the 20-HO-position of CPT was bound directly to the carboxylic acid groups of polyglutamate have been stopped or put on hold. Two novel camptothecin polymer conjugates, however, IT-101 and XMT-1001 are meanwhile in clinical development (see articles by Cissy Young et al. as well as Robert J Fram and Alex Yurkovetskiy). A further prodrug of a camptothecin, NKTR-102, a 4-arm pegylated form of irinotecan by Netktar, Inc. is under phase II clinical studies against colon and ovarian cancer and recently showed encouraging response data in platinum refractory ovarian cancer patients (J. Clin. Oncol., 28, 7S - suppl. Abstr. 5013, ASCO2010), but appears to act as a low-molecular-weight prodrug with improved pharmacokinetics and will not be discussed in this edition (see http://www.nektar.com/product_pipeline for further details on this technology)....

EZN-2208 (PEG-SN38) is a polymeric prodrug of anticancer agent SN38 (7-ethyl-10-hydroxyl-camptothecin) which is prepared by coupling a 40 kDa, 4-arm polyethylene glycol (PEG) with SN38, through a glycine spacer. The coupling strategy was developed to selectively link the 20-OH group of SN38, thus stabilizing the E-ring of SN38 in the active lactone form. EZN-2208 markedly increased the solubility of SN38 by about 1,000 fold, thus allowing systemic administration of SN38. In preclinical studies, EZN-2208 had excellent anticancer efficacy in several human cancer models derived from solid tumors and hematological malignancies. It outperformed the prodrug irinotecan (CPT-11) in these studies, including tumor models that failed to respond to CPT-11. Associated with enhanced activity, it was found that EZN-2208 prolonged the circulation half-life of SN38 and increased tumor-site accumulation of both PEG conjugate and released free SN38 compared to CPT-11. These observations may explain the enhanced anti-angiogenic activity that has been found with EZN-2208 compared to CPT-11. In two completed phase I studies, EZN-2208 was well tolerated in patients with advanced malignancies. The dose-limiting toxicity (DLT) was febrile neutropenia with or without fever, unlike CPT-11, for which unpredictable diarrhea was the DLT. The maximum tolerated dose and recommended phase II dose of EZN-2208 have been identified. Stable disease, sometimes prolonged and associated with tumor shrinkage, was observed as the best response in Phase I. EZN-2208 is being evaluated in phase II clinical studies in patients with metastatic colorectal and breast cancers and a phase I/II study in pediatric patients with cancer.

CRLX101 (formerly IT-101) is a first-in-class nanopharmaceutical, currently in Phase 2a development, which has been developed by covalently conjugating camptothecin (CPT) to a linear, cyclodextrin-polyethylene glycol (CDPEG) co-polymer that self-assembles into nanoparticles. As a nanometer-scale drug carrier system, the cyclodextrin polymeric nanoparticle technology, referred to as “CDP”, has unique design features and capabilities. Specifically, CRLX101 preclinical and clinical data confirm that CDP can address not only solubility, formulation, toxicity, and pharmacokinetic challenges associated with administration of CPT, but more importantly, can impart unique biological properties that enhance CPT pharmacodynamics and efficacy.

XMT-1001 is a conjugate of camptothecin (CPT) and Fleximer®, a biodegradable stealth polymer (poly [1- hydroxymethylethylene hydroxymethylformal], also called PHF). CPT is covalently linked to PHF via a hydrolizable linker. The conjugation of CPT with PHF increases the solubility of CPT. XMT-1001 releases CPT via intermediates camptothecin-20-O-(N-succinimidoglycinate) (CPT-SI) and camptothecin-20-O-(N-succinamidoyl-glycinate) (CPT-SA) over an extended time interval. XMT-1001 has an improved therapeutic window compared to CPT and irinotecan in human tumor xenograft models. XMT-1001 pharmacokinetics and biodistribution studies in rodents demonstrated extended plasma and tissue/tumor exposure for conjugated CPT and small molecule drug release products. An ongoing Phase 1 study of XMT-1001 in patients with advanced solid tumors demonstrates that XMT-1001 can be safely administered to patients. While neutropenia has been observed, no patients, to date, have experienced severe diarrhea or hemorrhagic cystitis. Early pharmacokinetic findings confirm the formation of the expected release products and a favorable PK profile. Evidence of a partial response in one patient as well as prolonged stable disease in numerous patients was observed. One patient had evidence of tumor shrinkage at a dose well below the maximum tolerated dose (MTD). Further clinical development is under consideration after completion of the Phase 1 study.

ProLindac™ (AP5346) is a 25 kDa polymer delivery vehicle based on hydroxypropylmethacrylamide (HPMA) to which DACH (diaminocyclohexane) platinum is bound. DACH platinum is the active moiety in the approved chemotherapeutic, oxaliplatin. The AP5346 polymer prodrug is currently in phase II clinical development. The polymer targets the drug to tumors and a pH-sensitive linker releases platinum more rapidly in low pH environments, as found typically in many tumors. ProLindac has completed a Phase I study in solid tumor cancer patients and has recently completed a Phase I/II monotherapy study in patients with recurrent ovarian cancer. With promising efficacy and safety data from these studies, clinical studies of ProLindac used in combination with other chemotherapeutic agents, including paclitaxel, are planned.

Hyaluronan receptors (CD44, RHAMM) are over-expressed in a wide variety of cancers including bladder and ovarian tumors. Based on this rationale, a hyaluronic acid paclitaxel conjugate, ONCOFID™-P, was developed for the treatment of refractory bladder cancer and peritoneal carcinosis. ONCOFID™-P has demonstrated improved in vitro and in vivo efficacy in hyaluronan receptor-positive tumor models and a favorable toxicity profile in rodents. It has been investigated in 15 patients with refractory bladder cancer by intravesical instillation, showing promising antitumor efficacy with 60 % of treated patients achieving a complete response. As a consequence, phase II studies with ONCOFID™-P are ongoing in six European countries. In addition, phase I studies have been initiated investigating the maximum tolerated dose (MTD) and the safety profile of ONCOFID™-P following IP infusion in patients affected by intraperitoneal carcinosis in ovarian, breast, stomach, bladder and colon cancer.

The (6-maleimidocaproyl)hydrazone derivative of doxorubicin (INNO-206, formerly DOXO-EMCH) is a prodrug of the anticancer agent doxorubicin which is selectively bound to the cysteine-34 position of endogenous albumin within a few minutes after intravenous administration planned for 2011. Preclinically as well as clinically, the albuminbound form of INNO-206 has a large AUC, a small volume of distribution and low clearance compared to doxorubicin, uptake in solid tumors being mediated by the pathophysiology of tumor tissue, characterized by angiogenesis, hypervasculature, a defective vascular architecture, and an impaired lymphatic drainage. The prodrug contains an acid-sensitive hydrazone linker allowing doxorubicin to be released either extracellularly in the slightly acidic environment often present in tumor tissue or intracellularly in acidic endosomal or lysosomal compartments after cellular uptake of the albumin conjugate by the tumor cell. INNO-206 shows significantly superior antitumor efficacy over free doxorubicin in a spectrum of preclinical tumor models. In a phase I study, INNO-206 showed a good safety profile at doses up to 260 mg/m2 doxorubicin equivalents. Although not the primary end point of the phase I study, INNO-206 was able to induce tumor regressions in breast cancer, small cell lung cancer and sarcoma. Phase II studies against gastric cancer, pancreatic cancer and sarcoma are planned for the end of 2010.

Urinary tract infections (UTIs) represent a recurrent health problem especially for women. More than 50% of women will suffer from a UTI at least once in their lifetime. Cranberries have long been used for their beneficial effects in preventing symptomatic UTIs in several published studies. However, cranberry products used in these clinical studies do not indicate the amount of active ingredients delivered that help to prevent UTIs. Therefore, a dose-dependent study was designed to understand the impact and safety profile of a standardized cranberry product (Proanthocyanidins Standardized Whole Cranberry Powder,PS-WCP) on reducing the recurrences of symptomatic UTI in culture-positive subjects. A 90- day randomized clinical trial including an untreated control group with a total of 60 female subjects between 18-40 years of age was conducted. Study subjects were randomly selected and assigned to three groups including an untreated control group (n=16), a low dose (500 mg daily, n=21) and a high dose (1000 mg daily, n=23) treatment group. The safety of PSWCP was assessed by evaluation of biochemical and hematological parameters on days 10, 30, 60 and 90 during the study, comparing the values with those at the baseline. Occurrence of UTI at baseline and during the follow-up period was characterized by the presence of symptoms and Escherichia coli in the culture of urine samples. The statistical analysis used was ANOVA. At the end of the 90-day treatment period, no significant changes were observed in the hematological and serum biochemical parameters. At the end of the study, change in the presence of E. coli in the untreated control group was not significant (p=0.7234), whereas, there was significant reduction (p<0.05) in the subjects positive for E. coli in both the high dose and low dose treatment groups, compared to baseline evaluation. Symptomatic relief was also reported in the low and high dose treatment groups, while none was reported by subjects in the untreated control group. In conclusion, PS-WCP was effective in safely reducing the number of E. coli positive subjects at both the 500 mg and 1000 mg dose levels and in ameliorating the symptoms of UTI in these subjects. Therefore, a daily dose of 500 mg or 1000 mg of PS-WCP may be considered as an adjunct to antibiotic prophylactic therapy against recurrent UTIs.

Triterpenoid alkaloids are one of the most important classes that have been studied. Buxus alkaloids are such a type of triterpenoid alkaloids of cycloartenol skeleton with a degraded C-20 side chain, and with the nitrogen containing substituents at C-3 and/or C-20. 219 Buxus alkaloids have been reported so far. This review article focuses on the structural characters, plant sources and biological activities of Buxus alkaloids. This review includes anti-myocardial ischemia, inhibition of acetyl- and butyryl-cholinesterases, cytotoxicity and antibacterial activities. The biological activities, mechanisms and structure modifications of cyclovirobuxine D (CVB-D) are also described.