Richard B. Greenwald

Effective drug delivery by PEGylated drug conjugates.

The current review presents an update of drug delivery using poly(ethylene glycol) (PEG), that focuses on recent developments in both protein and organic drugs. Certainly the past 10 years has resulted in a renaissance of the field of PEG drug conjugates, initiated by the use of higher molecular weight PEGs (M(w)>20,000), especially 40,000 which is estimated to have a plasma circulating t(1/2) of approximately 10 h in mice. This recent resuscitation of small organic molecule delivery by high molecular weight PEG conjugates was founded on meaningful in vivo testing using established tumor models, and has led to a clinical candidate, PEG-camptothecin (PROTHECAN), an ester based prodrug currently in phase II trials. Additional applications of high molecular weight PEG prodrug strategies to amino containing drugs are presented: similar tripartate systems based on lower M(w) PEG and their use with proteins is expounded on. The modification of a benzyl elimination tripartate prodrug specific for mercaptans is presented, and its successful application to 6-mercaptopurine giving a water soluble formulation is discussed. Recent novel PEG oligonucleotides and immunoconjugates are also covered. Clinical results of FDA approved PEGylated proteins are also presented.

PEG drugs: an overview

No low molecular weight (<20000) poly(ethylene glycol) (PEG) small molecule drug conjugates, prepared over a 20-year period, have led to a clinically approved product. In this area, published studies for these types of compounds have been scrutinized and their properties compared and contrasted to higher molecular weight conjugates where, during the past 5 years, a renaissance in the field of PEG (anticancer) drug conjugates has taken place. This new development has been attributed to the use of higher molecular weight PEGs (>20000), and especially employing PEG 40000 which is estimated to have a plasma circulating half life of approximately 8-9 h in the mouse. This recent resuscitation of small organic molecule delivery by high molecular weight PEG conjugates was founded on meaningful in vivo testing using established tumor models, and has led to a clinical candidate. Recent applications of high molecular weight PEG prodrug strategies to amino containing drugs are also detailed, and potential applications to proteins are proposed.

Camptothecin-20-PEG ester transport forms: the effect of spacer groups on antitumor activity.

An improved synthesis of the hindered PEG-camptothecin diester transport form has been achieved using the Mukaiyama reagent. We have also assessed the effect of changing the electronic configuration of the (d-position of PEG-camptothecin transport forms on the rates of hydrolysis of the pro-moiety, and attempted to correlate these differences to efficacy in two animal models. In addition to the simple substitution of N for O, other synthetic modifications of these atoms were accomplished by employing heterobifunctional linker groups. The half lives by disappearance (rates of hydrolysis) of the transport forms in buffer and rat plasma were determined. It was established that anchimeric assistance to hydrolytic breakdown of the pro-moiety occurs in a predictable manner for some of these compounds. Results for the new derivatives in a P388 murine leukemic model and HT-29 human colorectal xenograft study are also presented. The use of a glycine linker group was found to provide similar efficacy in rodent models to that of simple camptothecin 20-PEG ester, and displayed enhanced pharmacokinetics.

Highly water soluble taxol derivatives: 2′-polyethyleneglycol esters as potential prodrugs

Abstract 2′ and 7-polyethylene glycol esters of taxol were prepared and found to be essentially water soluble. The rates of hydrolysis of these compounds were measured under neutral, acidic and physiological (basic) conditions. The half-lives of O and N substituted 2′-esters are short enough to permit their use as produgs.

Anticancer drug delivery systems: multi-loaded N4-acyl poly(ethylene glycol) prodrugs of ara-C. II. Efficacy in ascites and solid tumors.

The synthesis of branched PEG (40,000) acids has been achieved using aspartic acid (Asp) and AspAsp dendrons. Complete conjugation of these dendritic acids with cytosine arabinoside (ara-C) was achieved by the use of spacers that allowed a greater separation of the branches to accommodate several large ara-C molecules in proximity to each other. The tetrameric and octameric PEG-ara-C amide prodrugs were much more effective in the treatment of solid and ascites tumors compared to the native drug. The greater loading of the PEG backbone appears to have achieved a minimum threshold concentration for the therapeutic delivery of ara-C.

Anticancer drug delivery systems: N4-acyl poly(ethyleneglycol) prodrugs of ara-C. I. Efficacy in solid tumors.

A systematic study of N(4) amino PEG-prodrugs of ara-C (1) was conducted and provided a series of disubstituted amides, as well as a carbamate derivative. These conjugates showed hydrolysis half lives in rat plasma from about 1 h to 3 days, but were stable for >24 h in phosphate buffer, pH 7.4. In an LX-1 solid lung tumor model some of the PEG prodrugs exhibited superior activity to ara-C when compared on a molar basis. One problematic issue that was identified in this investigation was the need to increase the loading of ara-C onto PEG in order to avoid highly viscous solutions.

Drug delivery of anticancer agents: water soluble 4-poly (ethylene glycol) derivatives of the lignan, podophyllotoxin.

This paper reports on the synthesis and in vivo oncolytic activity of a series of water-soluble acyl derivatives of polyethylene glycol (PEG) conjugated podophyllotoxin. Some analogs of the polymer conjugate showed significantly better activity in a murine leukemia model than native podophyllotoxin suspended in an intralipid emulsion. Additionally, when tested intravenously against a solid lung tumor (A549) model, some conjugated analogs were equivalent to the podophyllotoxin/intralipid emulsion, while those compounds demonstrating slower rates of plasma hydrolysis (in vitro) appeared to cause greater toxicity. There appeared to be an overall correlation between the in vivo antitumor activity of the conjugate and its rate of hydrolysis in vitro, with those showing faster release possessing greater antitumor activity. In conclusion, the solubilization and predictable release of podophyllotoxin from a PEG carrier was achieved and resulted in some derivatives demonstrating, at a minimum, equivalency with podophyllotoxin when administered on an equal molar basis. Further studies may be warranted to assess the PEG-conjugates pharmacokinetics and therapeutic indices in leukemic models.

Drug delivery systems: anticancer prodrugs and their polymeric conjugates

Prodrugs and their polymeric conjugates (transport forms) offer many promising modalities for new cancer chemotherapies. They present unique opportunities to increase the therapeutic index of a particular drug by exploiting the properties associated with the disease in a predictable fashion. Drugs can be chemically modified with a promoiety to achieve an exclusive selectivity by utilising specific enzymes, fenestrated vasculatures, more acidic environments and other unique properties of the tumour. Additionally, by incorporating a promoiety into existing immunoconjugates, greater efficacy can be achieved.

Tripartate poly(ethylene glycol) prodrugs of the open lactone form of camptothecin.

Two PEG prodrugs utilizing conjugation of PEG through the C-21 acid functionality as well as the C-17 OH group of CPT hydroxy-amide open forms were synthesized and characterized. Both of these open lactone tripartate prodrugs were shown to be water soluble and highly effective in MX-1 mouse xenograph studies. Indirect evidence implies that the initial ester or carbonate bond breaking is esterase mediated in the first step of the cascade of CPT release.

Stereoselective acylation of 20-(S)-camptothecin with amino acid derivatives using scandium triflate/DMAP

Abstract Facile esterification of the hindered 3° alcohol 20-( S )-camptothecin with Boc protected chiral amino acids or derivatives under mild conditions has been achieved in high yield using the combination of Sc(OTf) 3 and DMAP. The isomeric purity of the product was maintained under these conditions.