Amine-Functionalized, Water-Soluble Polyamides as Drug Carriers

Chapter 25

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 10, 2014 | http://pubs.acs.org Publication Date: July 18, 1991 | doi: 10.1021/bk-1991-0467.ch025

Amine-Functionalized, Water-Soluble as Drug Carriers

Polyamides

Eberhard W. Neuse and Axel G. Perlwitz Department of Chemistry, University of the Witwatersrand, WITS 2050, Republic of South Africa

Solubility in aqueous media is a prerequisite for the efficacious drug carrier action of polymeric carrier molecules designed for the reversible binding of certain pharmacologically active agents requiring intravenous or intracavitary administration in clinical use. The macromolecular carriers discussed in this communication are aliphatic polyamides possessing intrachain—type or side chain—attached, primary or secondary amine functions capable of drug binding. The polymers are perfectly soluble in water, which permits a rough fractionation by dialysis. The products retained in membrane tubing with 12000 — 14000 molecular—mass cut—off have inherent viscosities of 5-20 mL g . Several side—chain modification and model drug anchoring reactions are described, all leading to water-soluble product polymers. Notable among these are conjugates with organoiron (ferrocene) or platinum coordination complexes as examples of the pharmacologically important class of metal—containing polymeric drugs. -1

For the efficacious administration of polymer-bound drugs by intravenous or intracavitary (e.g. intraperitoneal) methods, smooth solubility of the conjugate in water is a desirable and frequently crucial prerequisite. Such solubility behavior will enable the conjugate to be distributed rapidly in the body's central circulation system and thus will facilitate transport to the target tissue. This feature will be of particular benefit with conjugates comprising lipophilic drug molecules unable per se to dissolve efficaciously in aqueous media. Polyethyleneimines, polyacrylates and other vinyl—type polymers have been used in numerous investigations as water-soluble drug carriers. However, problems of nonbiodegradability, frequently coupled with toxicity, would tend to militate against the use of these polymer types as components of pharmaceutical preparations. Heterochain-type polymers comprising amide, ester, or carbohydrate ether links in the backbone may provide the required biodegradability and thus would appear to offer advantages in the biomedical field. Polysaccharides, polypeptides and several 0097-6156/91/0467-0394$06.00/0 © 1991 American Chemical Society

In Water-Soluble Polymers; Shalaby, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


25. NEUSE & PERLWITZ

Polyamides as Drug Carriers

395

types of protein have indeed been used for drug anchoring, and there is room for challenging future developments, notably those concerning drug conjugates with immunoglobulins and antibodies possessing targeting functions. Some exellent discussions of these timely topics are on record (1-5). Problems of a different kind, however, have been associated with proteins as drug carriers insofar as enzymatic backbone degradation may now turn out to be too rapid for the carrier chain to maintain its operational integrity. In addition, most proteinaceous polymers are strongly immunogenic, in part owing to the large number of different amino acid constituents in the main chain, and are swiftly scavenged by the body's reticulo-endothelial system. Lastly, the reactive side groups in a protein intended to be utilized as drug-binding sites tend to be poorly described in terms of location and accessibility; as a result, the drug—coupling chemistry is frequently ill-defined and speculative, and well characterized, water-soluble protein-drug conjugates have rarely been isolated in the solid state. As part of a major program to develop more readily manageable, biologically functional polymer—drug conjugates distinguished by solubility in aqueous media, a large number of macromolecular carriers possessing amino, formyl, or carboxyl groups capable of reversible bond formation with biologically active agents have been synthesized in our laboratory. In the present article we describe three different synthetic approaches leading to linear polyamides with incorporated intra- or extrachain amino groups and discuss side-chain modification and model coupling reactions demonstrating the drug binding facilities of these target polymers. Structural Prerequisites For the structural design of macromolecular carriers intended for the synthesis of water-soluble polymer-drug conjugates a number of critical requirements must be considered, including the following: 1.

The polymeric backbone should be highly flexible as this will lead to an increase in the (positive) entropy of solution and thus render the dissolution process thermodynamically more favorable. In addition, it should incorporate an adequate number of groups or segments susceptible to solvation in aqueous medium, thus providing solubility in water. 2. The carrier molecule must comprise reactive functional groups as suitable binding sites for drug attachment. The groups should be separated from the main chain by short (5—15 constituent—atoms) side chains or spacers to diminish the steric inaccessibility caused by the polymeric backbone and should be of such a nature as to permit biological (i.e. hydrolytic and enzymatic) cleavage of the bonds generated in the drug coupling reaction. 3". Additional biofissionable functions required to facilitate drug release in the biological environment should be inserted into the spacer segments. At least one of these should be sufficiently remote from the main chain to allow for approach by proteolytic enzymes. 4. The carrier backbone should be biodegradable to permit catabolic elimination in the 'spent' state. To this end, amide or ester links susceptible to slow hydrolytic or enzyme-mediated cleavage should be incorporated into the main chain. 5. The carrier polymer must be nontoxic and should exert minimal immunogenicity. Synthetic oligo- and polypeptides are generally superior to natural proteins in this respect, notably if the number of amino acid monomers utilized in the synthesis is kept to a minimum.


396

WATER-SOLUBLE POLYMERS

The structural prerequisites enumerated in the foregoing were taken into account in the choice of the polyamide structures 1 - 3 here presented. The amino groups incorporated into these carrier polymers as drug binding sites offer several distinct preparative and functional advantages. Specifically:


1-.

2. 3.

Amino groups lend themselves to coupling reactions with carboxyl groups under mild conditions. Such coupling can often be carried out in aqueous or aqueous-organic solvents with the aid of techniques developed in amino acid chemistry. Being susceptible to protonation and hydrogen bonding, amino groups can function as hydrosolubilizing entities whenever present in excess over the number required for drug binding. Amino groups will render the polymers cationic by virtue of their susceptibility to protonation. In the treatment of malignancies, this may be of potential therapeutic benefit, many cationic polymers being known to show antineoplastic activity (6). In addition, improved pharmacokinetics may result from the conjugate's facilitated pinocytotic cell entry (7) and its enhanced attraction to cancerous cell tissue, the membrane surfaces of which are oftentimes more negatively charged than those of resting cells (8,9).

Synthesis of Carrier Polymers In the first synthetic approach, building up on the known (10) poly—D,L— succinimide synthesis by polycondensation of aspartic acid, copoly— merizations of D,L—aspartic acid with Νό-phthaloylornithine in various monomer feed ratios m/n were conducted in orthophosphoric acid at 185*C. This gave copoly(imide-amides), which upon hydrazinolysis in D M F suffered N-deprotection concomitantly with imide ring opening (Scheme 1). The resulting water-soluble target polymers, of the poly(a,/M),L-asparthydrazide-co-D,L-ornithine) type 1 (a form only shown here), possessed backbone compositions with the (randomly distributed) asparthydrazide and ornithine units present in x / y ratios higher by some 20-30% than the corresponding monomer feed ratios m/n (see Experimental Part). The powerful thermodynamic driving force for imide ring formation from the aspartic acid monomer can be expected to lead to preferential incorporation of succinimide units at the high polycondensation temperature of these experiments and thus may account for the observed differences in polymer composition and feed ratio. In the second approach, aliphatic diamines of the type N H 2 — C H ( C H ) C H 0 ( C H C H 2 0 ) C H 2 C H ( C H 3 ) - N H 2 ( ~ 20, 45) were copolymerized with iminodiacetic acid in polyphosphoric acid ( P P A ) at 140-170°C. This yielded polyiminodiacetamides of the general structure 2 (Scheme 2). The intrachain poly(ethyleneoxy) blocks in these carrier molecules act as the principal contributors to the compounds' excellent water solubility, and the secondary amino groups introduced as constituents of the iminodiacetic acid monomer provide the drug anchoring sites. The susceptibility of poly-D,L-succinimide to nucleophilic imide ring opening mediated by amines, affording N-substituted a,/?-D,L-aspartamide polymers, was utilized in the third synthetic approach. This ring opening reaction has been the subject of extended investigations (11-13), notably by Drobnik and Saudek with coworkers, and the drug carrier potential of polyaspartamides has been proficiently explored (13-16). In our laboratory, the reaction was found to proceed not only with monofunctional, but also, under carefully maintained experimental conditions, with difunctional amines 3

2

2

n

n





I—

NH, + NH. + χζ HOOC '

NHCO '

397

N H ^ COOH

N H ^ ^CO-'

(2)

2 as nucleophiles. This permitted the direct synthesis of polymers bearing free amino side groups without involvement of cumbersome N—protection and deprotection techniques (Scheme 3; a forms shown only). Diamines used included propylenediamine, N-(2-hydroxyethyl)ethylenediamine, diethylenetriamine, and hydrazine. The reactions were performed by treating the polysuccinimide with an excess of diamine in N,N-dimethylformamide solution. In a similar fashion, sequential treatment of the polyimide with a monofunctional and a difunctional nucleophile in given proportions afforded copolyamides composed in random sequence of one type of repeat units possessing a drug-binding primary or secondary amino group and another type of units bearing tertiary amino groups capable of providing hydro-


398

WATER-SOLUBLE POLYMERS

ν CO.

.NHv

(3) ^CONHj

CO


R

a: R =

^ y s N H

b: R =

^ / N H ^ ^ O H

c: R =

^v^NH.^sNHs

d: R =

-NH

2

2

solubility but unable to undergo simple drug coupling reactions (Scheme 4; a forms only). 1)

R'-NH

2)

R"-NH

DMF;

2

^ . C O

N

H

^^ C O ^ N H -

2

(4)

0-25°C

CO

^ C O N H

Jx+y

CONH

ir'

R'

4 a: R '

= ^ / ^ N ^ \ > ; R"

b: R '

= • x ^ w / ^ O

c: R '

=

d: R '

= χ\/-νΝΜβ2 ;

s^^NMe2;

; R" R"

^\ NH /sNH

R"

^ N H

x

s

2

2

1) = \ C O C I , O H " 2) H j N A ^ Û H CONH χ

H 0-CHCI ; H 0; 2

3

2

v

0-25 C

CONH

(5) :NCO.

NH HO

3b

5

HO

-'NH

5

OH

5

Polyamides 2, 3a, 3b, and 3d were further modified by side-chain N-acryloylation, and the acryloylated polymers, on treatment with ethanolamine, ethylenediamine, or propylenediamine in water, converted to side chain-extended, hydroxyl- or amine-functionalized derivatives



399


possessing the same solubility properties as their polymeric precursors. Polymer 5 derived from 3b exemplifies the products so obtained (Scheme 5; a form only). The polyamides 1—5 were purified by dialysis in aqueous solution. Freeze-drying provided the polymers in 25-70% yield as water-soluble solids.


Anchoring Reactions The drug-binding (anchoring) capabilities of the carrier polymers 1—4 were tested in a large variety of coupling reactions with suitably functionalized model compounds. In exemplifying experiments, copolymer 1 (x/y = 3.7) was treated with an excess of 5—(acryloylamino) salicylic acid in aqueous solution; this gave the water-soluble, polymeric salicylic acid derivative 6 (x/y = 3.7) via Michael addition of primary amino groups in 1 across the activated double bond in the acryloyl compound. The same educt polymer 1 (x/y = 3.7), upon coupling with phenylacetic acid via the active N-hydroxysuccinimide ester of this acid in acetonitrile-water, converted to the conjugate 7 (x/y = 3.7). In an analogous reaction, 1 (x/y = 6.2) gave the conjugate 7 (x/y = 6.2). Treatment of copolyamide 4a (x/y = 2.34) with potassium tetrachloro-

6 platinate in aqueous solution led to Pt ethylenediamine ligand in the carrier polymer, platinum complex 8 (x/y = 2.34). In another conducted analogously, 4a (x/y = 3.0) gave the = 3.0). Reaction of 2—ferrocenylpropanoic acid,

8

7 complexation with the affording the polymeric exemplifying experiment polymer complex 9 (x/y via its active hydroxy—

9

succinimide ester, with copolyamide 4d (x/y = 1.5) in aqueous tetrahydrofuran gave the ferrocene-containing conjugate 10 (x/y = 1.5), in which the proportion of ζ varied according to the employed reactant ratio. All conjugates were purified by dialysis and isolated by freeze-drying in yields of 35-70%. Conjugates with y (ζ in 10)

Amine-Functionalized, Water-Soluble Polyamides as Drug Carriers

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