METHODS OF IDENTIFYING BIOLOGICAL AGENT COMPOSITIONS
This application claims the benefit of U.S. provisional application no. 60/055,256 filed August 8, 1998.
FIELD OF THE INVENTION
The present invention relates to novel methods of identifying biological agent compositions useful in pharmaceutical, biopharmaceutical. diagnostic, imaging, immunology, veterinary, and agricultural applications.
BACKGROUND OF THE INVENTION
The conventional design of a new drug is very difficult. It demands design or discovery of a new molecule which precisely matches its molecular target. Moreover, once such a molecule is discovered, the new drug candidate must be soluble, bioavailable, resistant to metabolic enzymes, and be nontoxic to the patient. Modifications of the new molecule, necessary to satisfy the above requirements, too often negatively affect its therapeutic efficacy. Due to enormous complexity, producing a new drug takes a very long time and requires huge financial resources.
Recent advances in combinatorial chemistry technologies have allowed for faster throughput in the design of new molecules. This development markedly reduces the time and cost in designing a desired molecule. However, the problem of modifying such a molecule so that it is soluble, bioavailable. resistant to metabolic enzymes, and capable of penetrating through membranes, often remains unsolved.
The drug delivery industry has addressed some of these problems, and as a result developed the ability to simplify new product development by incorporating drugs into a carrier. For drug delivery assisted products, the time of development is shortened to seven years, and the average cost is brought down significantly.
Unfortunately, most drug delivery systems have several serious limitations. First,
they are able to solve only a limited number of aforementioned problems, and second they are not applicable to many drugs.
SUMMARY OF THE INVENTION
This invention is in the area of "combinatorial drug delivery" or "combinatorial formulation"'. The invention provides for a method of identifying a biological agent composition that can be applied to pharmaceutics, biopharmaceutics. diagnostics and imaging, immunology, veterinary, agriculture, and other areas where the properties of biological agents exhibited during interaction with a living organism or cell can be improved. Many biological agents are suitable, including those useful for diagnostics or imaging, or those that can act on a cell, organ or organism to create a change in the functioning of the cell, organ or organism. This includes, but is not limited to, pharmaceutical agents, genes, vaccines, herbicides and the like.
The current invention provides a method of identifying a biological agent composition of choice to create a complex that will render a target molecule soluble, bioavailable, resistant to metabolic enzymes, non-toxic, and freely traveling through membranes and into cells. By using segmented copolymers, such as for example, block copolymers, and preparing libraries of biological agent compositions, the invention has the ability to rapidly complex and identify the compositions of biological agents with desired biological properties. This invention can be applied in combination with high throughput screening of actual composition libraries, and can utilize mathematical concepts, which have been found to be beneficial in Combinatorial Chemistry.
The invention reduces the time and cost for creating desired drug compounds, which are not only immediately ready for clinical trials, but also possess a number of important characteristics increasing the probability of the ultimate success. Unlike combinatorial chemistry, the invention does not discover new drug structures or alter the desirable drug's characteristics, but instead provides optimal compositions of a
desired drug, solving the drug's problems of solubility, bioavailability, resistance to metabolic enzymes, toxicity, membrane transport, site specific delivery, and the like. Using a biological agent molecule as a starting point, the invention identifies new compositions with characteristics sought for the optimal performance of the selected molecule.
The invention thus relates to new methods of identifying biological agent compositions. The process involves preparing a plurality of compositions having segmented copolymers, wherein these segmented copolymers differ in at least one of their segment lengths, and then screening these compositions for the desired biological property or properties.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows identification of a biological agent composition with the desired (maximal) biological property (BP) using a library prepared based upon AnBm copolymers.
Figure 2 shows the relationship between CMC and pyrene partitioning coefficient, P, for pluronic block copolymers having varying lengths of ethylene oxide and propylene oxide segments for Pluronic L121. LI 01, LI 27, LI 23, LI 04, F108, L81. P85. P84. L61, L64. F87. L31, and F68 (37°C).
Figure 3 is a schematic of a high throughput screening procedure exemplifying the relationship between computerized analysis ("computations"') using a virtual base of polymer segments ("blocks") and segmented copolymers ("carrier molecules"), chemical synthesis of new perspective carrier molecules, preparing library of biological agent compositions ("complexing"), screening selected promising compositions using physicochemical and biological analysis and identification of a biological agent composition with desired properties.
Figure 4 shows a cycle of identifying a biological agent composition of choice through the use of a parent database of carriers, computational analysis, chemical synthesis, preparation and testing of a real library carriers (base of copolymers) and biological agent compositions, and identification of a composition with desired properties ("optimized formulation").
The schematic presentations in Figure 3 and 4 serve only as examples of possible screening and identification procedures pursuant to the invention. Particularly, the elements and sequence of steps of these procedures can be varied to accommodate the properties of a certain biological agent, drug candidate and/or drug delivery situation.
DETAILED DESCRIPTION OF THE INVENTION
Definitions
'Analysis: The review and classification of data obtained from testing the compositions using high throughput screening (or otherwise) to draw conclusions from the classified data. Analysis identifies the compositions with desired biological properties answering to a set of criteria including but not limited to the following: (/) whether any of compositions are good enough to be a final product, and {ii) whether the data from the testing supports creation of a new library for a new testing cycle.
•Architecture: Refers to copolymers having the same or similar formula, but with different methods of joining each of the polymer segments.
'Basis of copolymers: A plurality of segmented copolymers differing in at least one of their segment lengths, molecular architecture, or chemical structure.
'Biological agent: An agent that is useful for diagnosing or imaging or that can act on a cell, organ or organism, including but not limited to drugs {i.e., pharmaceuticals) to create a change in the functioning of the cell, organ or organism. Such agents can include, but are not limited to nucleic acids, polynucleotides.
antibacterial agents, antiviral agents, antifungal agents, anti-parasitic agents, tumoricidal or anti-cancer agents, proteins, toxins, enzymes, hormones, neurotransmitters, glycoproteins, immunoglobulins, immunomodulators, dyes, radiolabels, radio-opaque compounds, fluorescent compounds, polysaccharides. cell receptor binding molecules, anti-inflammatories, anti-glaucomic agents, mydriatic compounds, and local anesthetics.
'Biological property: Any property of a biological agent or biological agent composition that affects the action of the biological agent or biological agent composition during interaction with a biological system. This includes solubility, stability, analysis of spectral properties, binding with plasma proteins. DNA, RNA, specific receptors, enzymes or other molecules, resistance to metabolic enzymes, chemical stability, toxicity, membrane transport, of transport into, out of. within and through target cells, tissues or organs, site specific delivery, specific enzymatic activities, activation or suppression of gene expression, total DNA, RNA and protein biosynthesis, cell proliferation and differentiation, apoptosis. hormone and polypeptide secretion, bioavailability, pharmacokinetics. pharmacodynamics, efficacy, toxicity, therapeutic index and the like.
'Carrier: Segmented copolymers. and mixtures thereof including mixtures with other segmented copolymers, homopolymers. biological agents, and surfactants.
'Computational analysis: A computer program which analyzes structures of new carriers in a virtual library, predicts their interaction with the drug candidate, and selects the most promising carriers.
•Drug candidate: A substance with biological activity potentially useful for therapy. For the new composition development, the drug candidate can be used as a chemical substance or theoretical model defining the molecular structure and known properties including physicochemical properties and biological activity, mechanism
of action, disease target, initial screening results, and known or expected problems with pharmaceutical application.
'High throughput screening: Use of the set of analytical methods and procedures to test the properties of the library of the biological agent compositions. This includes screening of the composition using a biological model, for example, cell, animal or plant, measurement of physicochemical property, computational analysis, and the like.
'Library of biological agent compositions: A plurality of compositions of biological agents with carriers.
'Parent database of carriers: A computer database containing information on known drug carriers which includes (but is not limited to) at least one of the following: structure of the carrier molecules (segmented copolymers), structure and properties of its building blocks (segments), molecular architecture, and available data on properties of compositions of these carriers with various molecules, including physicochemical properties and biological activity, mechanism of action, disease target, initial screening results, and known or expected problems with pharmaceutical application.
'Preparing composition: Creation of compositions of a biological agent including drug candidates with a carrier. This includes mixing of the biological agent and the carrier under specific conditions of solvent composition, concentration, pH, temperature and the like as well as creation of computer database including parent database of carriers and information on biological agent including but not limited to chemical structure. This database can also contain available data on properties of biological agents, such as physicochemical properties, biological activity, and mechanism of action, disease target, initial screening results, and known or expected problems with pharmaceutical application.
'Segmented copolymer: A conjugate of at least two different polymer segments.
'Surfactant: A surface active agent that is adsorbed at interface.
♦Testing composition: Evaluation of the properties of composition using a biological model, including but not limited to cell, animal or plant models, measurement of physicochemical property of composition, and computational analysis.
♦Virtual library: A list of carriers potentially useful to the drug candidate.
The invention allows rapid selection and design of a carrier to meet specific delivery and efficacy criteria. The invention may incorporate the use of, (/') parental databases having a large number of chemical templates, ( /') exploratory virtual libraries or carriers, ( ) computational analysis for predicting chemical and physical properties of the complexed compounds, (z'v) validated solid-phase and solution-phase chemistries, and (v) testing.
In one embodiment, the invention relates to methods of identifying biological agent compositions of choice comprising:
(a) preparing a plurality of segmented copolymers. the segmented copolymers differing in at least one of the following, (/') at least one of their segment lengths, ( ) chemical structure. {Hi) copolymer architecture;
(b) preparing compositions of the segmented copolymers with at least one biological agent;
(c) testing at least one of the compositions of segmented copolymers with a biological agent for biological properties using a cell, animal, plant or other biological model, or measurement of a chemical or physical property in a test tube, or a theoretical model; and
(d) identifying the compositions with desired biological properties.
In another preferred embodiment, the segmented copolymer has at least one hydrophilic nonionic polymer and at least one hydrophobic nonionic segment. In another preferred embodiment, the segmented polymers have at least one cationic segment and at least one nonionic segment. In yet another preferred embodiment, the segmented polymers have at least one anionic segment and at least one nonionic segment. Also preferred are compositions where the segmented polymers have at least one polynucleotide segment and at least one segment which is not a nucleic acid. Further preferred are compositions where the polymer segments comprise at least one polypeptide segment and at least one non-peptide polymer segment.
The biological agent is an agent which is useful for diagnostics or imaging or that can act on a cell, organ or organism to create a change in the functioning of the cell, organ or organism. This includes, but is not limited to, pharmaceutical agents, genes, vaccines, herbicides and the like.
The term "preparing" is used in the broad sense to include design of theoretical models for computational analysis. The invention does not require that all or even any of segmented copolymers are synthesized, or that all or any of pharmaceutical compositions are actually prepared. The plurality of segmented copolymers can be constructed "on paper" and then tested in a computer database. Similarly, the compositions of the segmented copolymers with biological agents can be presented as a database.
The "testing" can be done using a variety of computational methods, so that part of or all process of the identification can be carried out or simulated virtually {i.e.. by computer).
The term "basis" refers to the plurality of segmented copolymers used in accordance with the invention.
The ability of segmented copolymers to form micelles, capture and release biological agents, interact with various systems of cell and organism affecting biological properties or otherwise interact modifying the biological response with respect to a certain biological agent depends upon the lengths (number of repeating units) in the segmented copolymer.
Within a "basis" of segmented copolymers, it has been discovered that there will be at least one copolymer which will form a composition with a biological agent exhibiting the desired biological properties. Because of the complex relationship between the biological properties and biological agent compositions, the discovery and optimization of useful biological agent compositions is ordinarily a very time- consuming process, requiring a high number of trials. The present invention provides a rational combinatorial method for identifying a useful biological agent composition by determining which copolymer of the basis possesses the desired properties with a specific biological agent.
In a preferred embodiment, the segmented copolymers have two segments having chemically different repeating units, designated "A" type and "B" type. The length of the A-type segment is designated as "«" and the length of B-type segment is "m". The length of the segment can be defined as its molecular mass or polymerization degree, or number of repeating units, atoms or the like. For example, if the length of the B-type segment is determined as a molecular mass of this segment, Mb, then the length of the A-type segment is calculated as follows:
M a= Mh D m p-p
(1) wherein P is the weight percentage of B-type segment in the block copolymer.
Assuming for simplicity that m and n designate the number of repeating units of the B- and A-type segments, and m and n can vary from 1 to N independently of each
other, this will produce a base of N2 segmented copolymers having a general formula AnBm. This base can be presented as a plurality of points in the 2-dimensional (2D) coordinate system {n, m}.
In accordance with the present invention, the library of biological agent compositions is tested for a useful biological property ("BP"). The BP can be any biological property including a drug therapeutic index, protein expression for a gene, immune response for a vaccine, or the like. The plurality of biological properties of the compositions within a composition library can be presented as a plurality of points in a three-dimensional space defined by a three-dimensional (3D) coordinate system {n, m, BP}. The multi-dimensional space corresponding to the library of biological agent compositions and certain biological property is termed herein as "the composition space." For example, using the base of AnBm diblock copolymers, the plurality of biological properties represent a three-dimensional network. The problem of identifying the optimal (for example maximal) biological parameter by testing the composition library is in essence equivalent to the mathematical problem of determining the extremum on a network (see Figure 1).
Computational and semi-computational methods have been developed to solve such problems (see. for example, G.J. Borse, Numerical Methods With Matlab: A Resource for Scientists and Engineers (1997); A. Dolan, J. Aldous.. Networks and Algorithms: An Introductory Approach (1994). These methods provide for "rational testing", so that the number of compositions that have to be tested to identify the desired biological property is substantially less that the total number of compositions in the library. For example, in the composition library on the base of AnBm copolymers the total number of compositions equals N . The number of tests to be performed with this library to identify optimal biological agent composition approximates N. Using data from independent experiments, which are collectively called herein the "clues", one can further decrease the number of the tests to be performed.
The segmented copolymers of the present invention can contain more than two segments. The lengths of the copolymer segments and the BP will provide variables in the composition space. For a base of copolymers having k-segments. the composition space is presented as follows: {n n2 ...nk, BP}. Similarly, several segmented copolymers with different architecture can be used in one library. In this case, the type of copolymer architecture can provide an additional variable in the composition space. For example, if j-types of the block copolymer architectures defined as Tj are used, then the composition space is as follows: {n n2 ...nk, Tj, BP}. The identification of the useful biological agent composition of the present invention may require testing the compositions for more than one biological property. If one different biological property is to be tested, the composition space is as follows: {n,, n2 ...nk, r,. BP,, BP2... BP,}. Concentrations of segmented copolymers and biological agents in the library of biological compositions can be also varied. In this case the concentrations of the copolymer, cc, biological agent. cb, provide additional variables in the composition space: {n n2 ...nk, Tj, cc, cb, BP,. BP2... BP,}.
In another preferred embodiment, the biological agent compositions contain cationic, anionic or nonionic surfactants. In such cases, the concentrations of surfactant, (cs) provide additional variables in the composition space: {nb n2 ...nk, Tj. cc, cs, cb, BP BP2... BPi}. Instead of the concentration of the surfactant, the ratio of concentration of surfactants and segmented copolymers or similar dependent parameters can be used. The library can be designed using a set of homologous surfactants, for example, these differing in the length of hydrophobic groups, so that certain compositions in the library will differ from each other by type of the surfactant used. One example includes fatty acid soaps of saturated and unsaturated fatty acids. The block copolymer basis can be designed with fatty acids having varying lengths of the hydrocarbon tail (φ), degree of saturation (Δ), and position of unsaturated bonds (γ). All of these parameters will be additional variables in the composition space: {n n2 ...nk, Tj, φ, Δ, γ, cc, cs, cb, BP BP2... BP,}.
The composition space can be designed using parameters that are dependent on the parameters characterizing the copolymer base and composition library, such as (nb n2 ...nk, Tj, φ, Δ, γ, cc, cs, cb). It its believed, for example, that the critical micelle concentration (CMC) of certain amphiphilic segmented copolymers. such as poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) block copolymers (also known under a name Pluronics™), depends upon the length of polymer segments (See Kabanov. et al, Macromolecules, 28:2303-2314, 1995). Similarly, the partitioning coefficient of a drug in the block copolymer micelles. P, in certain cases depends on the length of the segments of this block copolymer (Kabanov, et al., Macromolecules, 28:2303-2314, 1995). The relationship between CMC and P is shown in Figure 2.
In some cases, it is beneficial to use such dependent parameters as CMC and P to design the composition space. One example is a library of compositions of a pharmaceutical drug {e.g., anticancer antibiotic, neuroleptic, anti-HIV drug or the like) with Pluronic block copolymers. In this case the copolymer concentration, c, serves as third useful parameter in the composition space. In the current example the concentration of Pluronic copolymers is varied from about 0.000001% (w/v) to about 10% (w/v) or the solubility limit of the given copolymer at 37°C. Therefore the composition space can be presented as: {CMC, P, c, BP}, where BP is a useful biological property (such as the therapeutic index).
Without wishing to be bound to a specific theory, one can exemplify the use of CMC and P in the composition space using Pluronic block copolymers. Pluronic micelles play an important role in certain biological agent compositions (Kabanov et al. FEBS Lett 258:343, 1989). Those micelles surround a drug with a biologically inert polymer shell, protect it while in the blood stream, deliver it to a target cell and dispense it to the target's intracellular compartment. In this way, the non-targeted cells are protected from the drug's potentially toxic effects (Kabanov et al., J. Contr. Release. 22:141, 1992). At the same time, the single chains of Pluronic block
copolymers (so-called "unimers"') inhibit certain drug efflux mechanisms resulting in increased cytotoxic activity of the drugs against multiple drug resistant (MDR) tumors (Alakhov et al. Bioconjugate Chemistry 7: 209 (1996); Venne et al. Cancer Research, 56:3626, (1996)). Similar mechanisms underlie the effects of Pluronic copolymers on drug transport across the blood-brain barrier and intestinal epithelium. By changing the lengths of the ethylene(oxide) and propylene(oxide) segments, one can select Pluronic copolymers which are more effective or less effective with respect to certain classes of cells and drug transport systems.
Therefore, the effects of block copolymers in biological agent compositions are two-fold. First, they form self-assembled drug carriers masking the drug from the undesired interactions in the body and providing for site specific drug delivery. Second, they act as modifiers of biological response with respect to a drug by affecting drug transport systems in the cell. These effects of block copolymer are related to the CMC and P.
Lower CMC and higher P indicate: (a) more stable micelles, (b) less block copolymer unimers available in solution, and (c) stronger attachment of the drug to micelle. These conditions correspond to the highest degrees of protection of the drug by the micelle from degradation and elimination by the body defense mechanisms. Also, the metabolism of the drug is minimal under these conditions and the drug toxic effects are minimized. However, the release of the drug from the micelles in the disease site is also minimal, thereby decreasing drug therapeutic effect. The concentration of the copolymer unimers is very low. which decreases the effects of the biological agent composition.
In contrast, when the CMCs are high and P is low, the micelles are very unstable, the concentration of unimers is high, and the drug is easily released in a free form. At the same time, high CMC is usually observed with hydrophilic block copolymers, which are also not active with respect to the drug transport systems. Further, since the drug is mainly in free form, it is also not protected from metabolic degradation,
and is more toxic than micelle-incorporated drugs. Therefore, with certain applications (for example anticancer antibiotics), selection of the optimal block copolymer composition yields Pluronic copolymers with intermediate CMC and P values. The CMC vs. P graph in Figure 2 provides a useful tool in the identification of such compositions with Pluronic block copolymers. It simplifies testing and identification procedures by using CMC vs. P isotherm instead of multidimensional space with the lengths copolymer segments as the coordinates. The information about the effects of the unimers and the micelles in drug actions provides the clues permitting to simplify the identification of the useful biological agent composition. Normally the clues are used to decrease the number of coordinates in the composition space as well as the number of compositions to be tested.
Segmented copolymers. The segmented copolymers of this invention are most simply defined as conjugates of at least two different polymer segments (see for example, Tirrel, Interactions of Surfactants with Polymers and Proteins, Goddard and Anantha-padmanabhan, Eds., pp. 59 et seq., CRC Press, Boca Raton, Ann Arbor, London, Tokyo, 1992). Some segmented copolymer architectures are presented below:
Diblock Triblock
- Λ O — cr\P — °~\ — σ u θL
Multiblock
(AB)„ Starblock A M,±B22,- Starblock
(The straight and wavy lines designate different polymer segments while the circles designate the links between these segments.)
The simplest segmented copolymer architecture contains two segments joined at their termini to give an A-B type diblock. Conjugation of more than two segments by their termini yields an A-B-A type triblock, ...ABAB... type multiblock, or even multisegment ...ABC... architectures. If a main chain in the segmented copolymer can be defined in which one or several repeating units are linked to different polymer segments, then the copolymer have a graft architecture, e.g., A(B)n type. More complex architectures include for example (AB)n or AnBm starblocks that have more than two polymer segments linked to a single center.
One method to produce segmented copolymers includes anionic polymerization with sequential addition of two monomers. See for example, Schmolka J., Am. Oil
Chem. Soc, 1977, 54:110; Wilczek-Vera et al., Macromolecules, 1996, 29:4036. This technique yields block copolymers with a narrow molecular mass distribution of the polymeric segments. Solid-phase synthesis of block copolymers has been developed recently that permit controlling the growth of the polymer segments with very high precision (Vinogradov et al, Bioconjugate Chemistry, 7:3, 1996). In some cases the block copolymers are synthesized by initiating polymerization of a polymer segment on ends of another polymer segment (Katayose and Kataoka. Proc. Intern. Symp. Control. Rel. Bioact. Materials, 1996, 23:899) or by conjugation of complete polymer segments (Kabanov et al, Bioconjugate Chem., 1995, 6:639: Wolfert et al, Human Gene Ther., 1996, 7:2123). Properties of block copolymers in relation to this invention are determined by (1) block copolymer architecture and (2) properties of the polymer segments. They are independent on the chemical structure of the links used for conjugation of these segments (see, e.g., Tirrel In Interactions of Surfactants with Polymers and Proteins, Goddard and Ananthapadmanabhan, Eds., pp. 59 et seq., CRC Press, Boca Raton, Ann Arbor, London, Tokyo, 1992; Sperling. Introduction to Physical Polymer Science, 2d edn., p. 46 et seq., John Wiley & Sons, New York, 1993).
Linking can be accomplished by a number of reactions, many of which have been described generally in conjugate chemistry. These can involve a terminal hydroxyl group on one polymer segment, e.g. , R -0-(C2H40)-H, in which R3 is hydrogen or a blocking group such as alkyl, and an appropriate group on another polymer segment, the two being joined directly or indirectly; i.e.. through a third component. Alternatively, a terminal group can be converted to some other functional group, for example amino, which then is allowed to react either with the next polymer segment or another linking component. The linking group thus may be formed either by reac- tively involving a terminal group of a polymer segment or by replacing the terminal group. For example, a carboxylic acid group can be activated with N,N'- dicyclohexylcarbodiimide and then allowed to react with an amino or hydroxy group to form an amide or ether respectively. Anhydrides and acid chlorides will produce
the same links with amines and alcohols. Alcohols can be activated by carbonyldiimidazole and then linked to amines to produce urethane linkages or activated to produce ethers or esters. Alkyl halides can be converted to amines or allowed to react with an amine, diamines, alcohols, or diol. A terminal hydroxy group can be oxidized to form the corresponding aldehyde or ketone. This aldehyde or ketone is then allowed to react with a precursor carrying a terminal amino group to form an imine which, in turn, is reduced, with (for example) sodium borohydrate to form the secondary amine. See Kabanov et al, J. Controlled Release, 22:141 (1992); Meth. Enzymol., XLVII, Hirs & Timasheff, Eds., Acad. Press, 1977. The linkage thereby formed is an -NH- group, replacing the terminal hydroxyl group of the polymer segment.
Alternatively, a terminal hydroxyl group on the polymer can be allowed to react with bromoacetyl chloride to form a bromoacetyl ester which in turn is allowed to react with an amine precursor to form the -NH-CH2-C(O)- linkage. Immobilized Enzymes, Berezin et al, Eds., MGU, Moscow, 1976, i.e., -NH-CH2-C(O)-. The bromoacetyl ester of a polymer segment also can be allowed to react with a diaminoalkane of the formula NH2-CqH2q-NH2 which in turn is allowed to react with an carboxy group on another polymer segment, or an activated derivative thereof such as an acid chloride or anhydride. The bromoacetyl ester also can be allowed to react with a cyanide salt to form a cyano intermediate. See, e.g., Sekiguchi et al, J. Biochem., 85, 75 (1979); Tuengler et al, Biochem. Biophys. Acta, 484, 1 (1977); Browne et al, BBRC, 67, 126 (1975); and Hunter et al, J.A.C.S., 84, 3491 (1962). This cyano intermediate then can be converted to an imido ester, for instance by treatment with a solution of methanol and hydrogen chloride, which is allowed to reacted with a amine precursor to form a -NH-C(NH2+)CH2C(O)- linkage. A terminal hydroxyl group also can be allowed to react with l,l'-carbonyl-bis- imidazole and this intermediate in turn allowed to react with an amino precursor to form a -NH-C(O)O- linkage. See Bartling et al, Nature, 243:342 (1973).
A terminal hydroxyl also can be allowed to react with a cyclic anhydride such as succinic anhydride to yield a half-ester which, in turn, is allowed to react with a precursor having terminal amonogroup using conventional condensation techniques for forming peptide bonds such as dicyclohexylcarbodiimide, diphenylchlorophos- phonate, or 2-chloro-4,6-dimethoxy-l,3,5-triazine. See e.g., Means et al, Chemical Modification of Proteins, Holden-Day (1971). Thus formed is the -NHC(O)- (CH2)qC(O)O- linkage.
A terminal hydroxyl group also can be allowed to react with 1 ,4-butanediol diglycidyl ether to form an intermediate having a terminal epoxide function linked to the polymer through an ether bond. The terminal epoxide function, in turn, is allowed to react with an amino precursor. Pitha et al. Eur. J. Biochem., 94:11 (1979); Elling and Kula, Biotech. Appl. Biochem.. 13:354 (1991); Stark and Holmberg, Biotech. Bioeng., 34:942 (1989).
Halogenation of a terminal hydroxyl group permits subsequent reaction with an alkanediamine such as 1,6-hexanediamine. The resulting product then is allowed to react with carbon disulfide in the presence of potassium hydroxide, followed by the addition of proprionyl chloride to generate a isothiocyanate which in turn is allowed to react with an amino precursor to yield a -N-C(S)-N-(CH2)6-NH- linkage. See Means et al, Chemical Modification of Proteins. Holden-Day (1971). The polymer chain terminating in an amino group also can be treated with phosgene and then another polymer segment containing amino group to form a urea linkage. See Means et al, Chemical Modification of Proteins, Holden-Day (1971).
The polymer segment terminating in an amino group also can be treated with dimethyl ester of an alkane dicarboxylic acid and the product allowed to react with an amino precursor to produce a -N-C(NH2+)-(CH2)4-C(NH2+)-N- linkage. See Lowe et al, Affinity Chromatography, Wiley & Sons (1974). The polymer segment terminating in an amino group can also be allowed to react with an alkanoic acid or fluorinated alkanoic acid, preferably an activated derivative thereof such as an acid