Pharmacological Research Communications, Vol. 7, No. 1, 1975 MACROMOLECULAR DRUGS ACTING AS PRECURSORS OF NON-MACROMOLECULAR
ACTIVE SUBSTANCES
-
PRELIMINARY
CONSIDERATIONS
Pao!o F e r r u t i - Ist. Chimica Industriale del Politecnico Sez. Chimtca Macromolecolare e Materiali P. za Leonardo da Vinci 32 - 20133 Milano, Italy
Received28 May 1974
Summary: Some principles at the basis of the potential i n t e r e s t of m a c r o m o l e c u l a r drugs are defined.
T h e main lines of a r e s e a r c h on purposely tailored synthetic
high p o l y m e r s acting as p r e c u r s o r s of n o n - m a c r o m o l e c u l a r active substances are given.
The chemical aspects of proper design of these p o l y m e r s a r e discussed
in detail.
The pharmacological application of purposely tailored synthetic high p o l y m e r s is at present a l a r g e l y unexploited field. Nevertheless,
ma
c r o m o l e c u l a r drugs appear to be of g r e a t actual or potential interest. The purpose of this communication is to define some principles at the basis of this interest and to give some p r e l i m i n a r y considerations on a r e s e a r c h line which is actually in p r o g r e s s , the f i r s t results of which will be given in the following papers of this series.
Ist pubblication of a collaborative r e s e a r c h project
with the Institute of
Pharmacology and Pharmacognosy, directed by P r o f e s s o r R. Paoletti
2
Pharmacological Research Communications,, Vol. 7, No, 1, 1975
_Forseea~b.le behavioral peculiaritles of macromolecular dru~ in bioI0gieal environments, .and their subdivision Generally speaking, it may be expected that a macromolecular substance compared to an analogous non-macromolecular one shows some substantial behavioral differences in biological environments. The potential possibilities of original applications of macromole cular drugs are based precisely on these forseeable differences linked to their macromolecular nature
(I)
.
The following outline is indicative on
this point : i) A high polymer injected into a living organism is practically non elimina ble through the usual excretion routes. To be eliminated it must first of all undergo a degradation process which brings it to. an average molecular weight lower than a certain threshold estimated at around 10,000 - 20, 000, as found experimentally, for instance, in the case of polyvinylpyrrolidone (2) The above obviously refers only to polymers injected into the body and not taken by other r o u t e s , for instance, o r a l l y .
In c o n t r a s t , in the l a t t e r c a s e s
the high p o l y m e r s p r e s u m a b l y r e m a i n confined to the digestive t r a c t and do not e n t e r the bloodst~ream u n l e s s they a r e b r o k e n down. 2) A m a c r o m o l e c u l a r substance m a y have a different localization in living o r g a n i s m s c o m p a r e d to a s i m i l a r non m a c r o m o l e c u l a r one.
In fact, as we
shall s e e l a t e r o n , high p o l y m e r s a r e capable of p e n e t r a t i n g into the c e l l s , o r at l e a s t into c e r t a i n types of c e l l s , by a specific m e c h a n i s m . F u r t h e r m o r e , it is always possible to a r r a n g e that high p o l y m e r s acting as p r e c u r s o r s of n o n - m a c r o m o l e c u l a r drugs have a coefficient of distribution between the v a r i o u s p h a s e s - e s p e c i a l l y between the aqueous and the lipid p h a s e - differing from that of the active compounds of which they a r e the p r e c u r s o r s . 3) A multifunctional m a c r o m o l e e u l e is usually much m o r e capable than its non m a c r o m o l e c u l a r homologues of i n t e r a c t i n g with other m a c r o m o l e c u l e s p r e s e n t in biological s y s t e m s ,
with m e m b r a n e s o r other cell s t r u c t u r e s , and also with
foreign s u b s t a n c e s accidentally introduced. Leaving out p o l y m e r u t i l i z e d o r t e s t e d as m a t e r i a l s for the c o n s t r u c tion of artificial p r o s t h e s e s ( p o l y m e r s of b i o m e d i c a l interest) which do not fall
Pharmacologica/ Research Communications, Vo/. 7, No. 1, 1975 within the concept of a drug, for the purposes of the present work m a c r o m o l e cular drugs may be divided into two categories: 1) M a c r o m o l e c u l a r substances which act because they are such and generally are not expected to require a change of structure to exert their action. 2) M a c r o m o l e c u l a r substances which act because they are p r e c u r s o r s of p h a r macologically active n o n - m a c r o m o l e c u l a r drugs. We have already p e r f o r m e d two studies in the field of m a c r o m o l e c u lar drugs, regarding substances belonging to tlm f i r s t group (maeromolecuIar drugs acting because they are such) accerdirg to the above classification. These studies deal with polymers having anti-heparin activity (3 - 6)and other with an t[n-,etastatic action (7). The present a study, however, deals with m a c r o m o l e cular drugs of the second group {precursors of active substances).
Main Purposes of the researc_h.on sy_nthetic highA3olymers acting as p r e c u r s o r s of non m a c r o m o I e c u l a r active substances This I'ine of r e s e a r c h proposes to utilize suitable m a c r o m o l e c u l a r m a t r i c e s as vehicles for pharmacologically active substances. This may be achieved by linking the pharmacologically active substances to convenient m a c r o m o l e c u l a r m a t r i c e s , preferably by means of covalent bonds p o s s e s s i n g limited stability in biological environments. This would lead to gradual and, in many cases, kinetically controlled r e l e a s e of the drug. This is an almost completely new field. Among previous r e f e r e n c e s mention may be made, as examples, to the p r e p a r a t i o n of p o l y m e r s of vinyl e s t e r s of salicylic or s i m i l a r acids
(8, 9), and of some derivatives of natural
polysaccharides with antibiotics and other drugs. (10). Reference has also been made, to be possibility of binding p h a r m a c a to m a c r o m o l e c u l a r m a t r i c e s by Batz, Franzmann, and Ringsdorf (11). The aim of this r e s e a r c h is to achieve one of the following r e s u l t s or even both simultaneously : a) Maintenance in time of suitable bloods levels of drugs which normally have an average life in the body too short to be of true therapeutic interest.
3
Pharmacological Research Communications, Vo/. 7, No. 1, 1975
4
b) P r e f e r e n t i a l localization and subsequent r e l e a s e of the drug at the level
of
given t a r g e t cells or groups of cells. E l e m e n t s exist which indicate that even this second point can be achieved in practice. During a now completed study on p o l y m e r s of antisilico ttgeutc action, we h a v e previously d e m o n s t r a t e d that polym~ers of poly-N-oxide type a r e actively concentrated in vivo at the level of the cells of the r e t i c u l o e n dothelial s y s t e m , by m e a n s of a specific m e c h a n i s m (plnocytosts)
(12, 13).
It
is obviuus that a drug whose site of action is expected, f o r instance; to be at the level of such cells is capabl e, at l e a s t in principle, of being conveyed into these cells by m e a n s of m a c r o m o l e c u l a r m a t r i c e s of the type indicated,
and
subsequently r e l e a s e d . This would lead to obvious advantages as r e g a r d s the effectivene~ss and selectivity of action of the drugs.
,_Chem_ical ~$pects of the d e s i g n of p o l y m e r s acting as p r e c u r s o r s . o f non m a c r p _m01eculax~ active substa:rmes The design of m a c r o m o l e c u l a r drugs acting by slow r e l e a s e of non m a c r o m o l e c u l a r active compounds, i n v o l v e s three main aspects;
1} the type
of the m a i n backbones of the m a c r o m o l e c u l a r c a r r i e r s of the active substances; 2) the chemical nature of the bonds with the active substances;
3) the n a t u r e of
the n o n - a c t i v e units, which, for solubilizing p u r p o s e s m u s t be usually added, e. g . , through copolymerization techniques. These t h r e e aspects will be d i s c u s s e d separately. 1) Types of main backbone Usually the t e r m "main backbone" of a m a c r o m o l e c u l e indicates the p a r t of a m a c r o m o l e c u l e r e m a i n i n g after r e m o v a l of the substituents : for example, in vinyl p o l y m e r s (I)
1 X
Pharmacological Research Communications, Vol. 7, No. 1, 1975 the m a i n b a c k b o n e is CH 2 - CHI
-] X
(if) c l o s e l y r e l a t e d to the polyvinylic backbone is the p o l y v i n y l i d e n i c backbone
(m) CH 2 - C I
I ' 1 X
(m) which m a y be c o n s i d e r e d m e r e l y a p a r t i c u l a r c a s e of the polyvinylicbackbone. F o r the p u r p o s e of o u r investigation, p o l y v i n y l i c or p o l y v i n y l i d e n i c b a c k b o n e s a p p e a r to b e v e r y c o n v e n i e n t for the following r e a s o n s : 1) the b a c k b o n e s t h e m s e l v e s a r e c h e m i c a l l y i n e r t and a r e not e x p e c t e d to r e a c t r e a d i l y with the n o r m a l c o m p o n e n t s of a b i o l o g i c a l e n v i r o n m e n t . 2) M o r e i n f o r m a t i o n is a v a i l a b l e on the s y n t h e s i s of p o l y m e r s o r c o p o I y m e r s of s u c h a s t r u c t u r e than of any o t h e r c l a s s of s y n t h e t i c p o l y m e r s . S e m i s y n t h e t i c p o l y m e r s which m a y be of i n t e r e s t could be a l s o d e r i v e d f r o m n a t u r a l l y o c c u r r i n g m a c r o m o l e c u l e s of p o l y - s a c c h a r i d e s t r u c t u r e , e.g. dextrans. P o l y m e r s with a l e s s c o m m o n type of m a i n b a c k b o n e , p o l y - a m i d o a m i n e s (IV), (V) : O
O li
tl 1 3 ~-CH OH C-N-R -N-C-CH CH,_-N-R -N-
or
0
0
CH CH C - N - R - N - C - C H 2 2 i I R R
(v)
2
CH-N2 12 R
x
5
6
Pharmacological Research Communications, VoL 7, No. 1, 1975
from a synthetic point of view appear well suitable for our investigation. P o l y m e r s of this class have been already investigated by our group as antihepartn (4, 6) and aatimetastatic (7) agents. The synthesis of these polymers can be c a r r i e d out under very mild conditions and in the presence of a very l a r g e number of other chemical functions in the side chains {14}; m o r e o v e r , the resulting polymers may be easily designed to be water or oil-soluble by varyl.ng the nature of the R radicals. However, in these polymers the m a c r o m o l e c u l a r backboz~,: should not be expected to be chemically inert, mainly because of the presence of basic t e r t i a r y amino groups. P o l y m e r s having other types of main backbone, e . g . p o l y e s t e r s , p o l y - a m i d e s , polyurethanes, show a low priority for our purpose, at least initially, mainly because the c h e m i c a l r e a c t i o n s involved in the synthesis of these polymers are not easily c a r r i e d out in the presence of reactive functions in the side chains. An important point is, if the main backbone of a hypotheti cal m a c r o m o l e c u l a r drug will be degraded in a biological environment. Most polysaccharides, such as dextrane, are metabolized in m a m m a l i a n organisms. P o l y - a m i d e - a m i n e s , on the other hand, are easily degraded in any aqueous environment even at neutral pH, by hydrolysis of the amide bonds, a reaction probably catalyz ed i n t r a m o l e e u l a r l y by the t e r t i a r y amino groups
(14}.
A poly-vinylic (or-vinylidenic) backbone, on the contrary, should be little, or not at all degraded in the body environment, whatever is the fate of the side-substituents.
This fact is best exemplified by the case of high molecu-
lar w e i g h t poly-N-vinylpyrrolidone, which r e m a i n s indefinitely in the body, without being eliminated, since no degradation occurs which brings the polymer below the renal threshold (2). On the other hand, this behaviour does not elicit obvious toxic side effects in man.
F o r m a c r o m o l e c u l a r drugs to be taken orally, the non-degradability of the main backbone is obviously of advantage, since only the active substance which is r e l e a s e d is absorbed. On the c o n t r a r y , if the m a c r o m o l e c u l a r drug has to be used,
for
Pharmacological Research Communications, Voi. 7, No. 7, 1975
7
instance, p a r e n t e r a l l y its main backbone m u s t be d e g r a d a b l e , in o r d e r to
avoid any long term permanence and possibly any accumulatlon in the body of macromolecular residues.
2) Binding of d r u g s t.o a m a c r o m o ! e c u l a l : back.b0ne Covalent bonds of l i m i t e d stability in biological e n v i r o n m e n t s a r e in our opinion the m o s t suitable ones for attaching a n o n - m a c r o m o l e c u l a r acti ve s u b s t a n c e to a m a c r o m o l e c u l a r backbone, when a slow and gradual r e l e a s e of this active subst,,mce in the body should be achieved. E s t e r bonds should be c o n s i d e r e d f i r s t , d r u g m o l e c u l e allows such a type of binding.
when the s t r u c t u r e of the
The e s t e r bonds can be m a d e m e
r e o r l e s s e a s i l y cleaved by v a r y i r g the s t r u c t u r e of the side groups in the m_a c r o m o l e c u l a r m a t r i c e . This
will be e x e m p l i f i e d in a following p a p e r of this
s e r i e s . O t h e r types of linkage, such as amidic or imino bonds, m a y also be suitable in p a r t i c u l a r c a s e s . The e s t e r bond may be utilized when the d r u g m o l e c u l e c o n t a i n s hydroxyl group or c a r b o x y l groups. In the f o r m e r c a s e , a way to obtain polym e r s o r c o p o l y m e r s containing the d e s i r e d d r u g - p r e c u r s o r units (V1) might be the s y n t h e s i s and the p o l y m e r i z a t i o n or the c o p o l y m e r i z a t i o n of, for instan_ ce, the a c r y l i c o r m e t h a c r y l i c e s t e r s of the d r u g i t s e l f (VII): -
HC1
AOH + C H 2 = CH-COC~
~ CH 2 = CH-COO-A
-CH2-CHI C~
I
(or o t h e r synthetic r e a c t i o n s )
VII
O
X
f
A
A = Drug r e s i d u e
VI
Simple as it m a y s e e m , this method has s e v e r a l d i s a d v a n t a g e s . F i r s t , other groups p r e s e n t in the drug m o l e c u l e m a y s e r i o u s l y i n t e r f e r e with the p o l y m e r i z a t i o n p r o c e s s .
Secondly, it is n e c e s s a r y to s y n t h e s i z e
and p o l y m e r i z e a new m o n o m e r i c d e r i v a t i v e for e v e r y active s u b s t a n c e
to
be bound to a m a e r o m o l e c u l a r backbone. S i m i l a r c o n s i d e r a t i o n s hold t r u e also when d r u g s containing amino groups a r e to be bound to a m a c r o m o l e c u Iar m a t r i c e with amidic bonds.
Pharmacological Research Communications, Vol. 7, No. 1, 1975
8
A much b e t t e r method (15)~'involves the synthesis of m o n o m e r s bearing
in the side chains suitable chemical functions, yielding the d e s i r e d
e s t e r (or amido) linkages. P o l y m e r s or c o p o l y m e r s obtained from these m o n o m e r s (VIII) could give directly the d e s i r e d end products (VI, IX) by t r e a t m e n t with the active substances to be bound.
.4-OH A-NH2
-oH2-I!:-o,
-CH2-CH-
+
CO I
B
X
A - drug residue B = leaving group r e s i d u e This method p r e s e n t s two significant advantages: first,, no i n t e r actions a r e expected to occur with many chemical functions which may be p r e s e n t in the drug molecule, as, for instance, with u n s a t u r a t e d s t r u c t u r e s ; secondly, a single " m o t h e r " p o l y m e r m~w be utilized to bind covalently a n u m b e r of active s u b s t a n c e s , provided they b e a r free hydroxyl or amino grouis, simply by t r e a t i n g this p o l y m e r with the active s u b s t a n c e s . Acryloyl and m e t h a c r y l o y l c h l o r i d e s do not appear a p p r o p r i a t e m o n o m e r s for this p u r p o s e , mainly b e c ~ s e of their high r e a c t i v i t y and lack of selectivity. Consequently, a study on other active derivatives of acrylic o r m e t h a c r y l i c acids is presently in p r o g r e s s . We have already obtained high p o l y m e r s of acrylic (X) and m e t h a c r y l i c (XI) e s t e r s of N-hydroxy-succ_i nimide (16):
CH2=~H CO f
0
X
?H 3 CH =C 2
CO
XI
Pharmacological Research Communications, Vol. 7, No. 1, 1975
9
The resulting p o l y m e r s a r e very good p r e c u r s o r s of p o l i a c r y l a m t des or polymethaerylamtdes, as indicated in the following synthesis of l i n e a r p o l y - N - a l l y l a c r y l a m i d e , which cannot be otherwise p r e p a r e d : N
m
-CII2-CHI
radtcal X initiators
a.Uylamine
-CH-CH21 CO
-C4115NO 3
NH
CO I
O O I
I
I
O
CH2-CH=CH 2 X
X
.
_
J
Other activated e s t e r of acrylic and m e t h a c r y l i c acids have been subsequently described (11).
The above p o l y m e r s , however, did not prove
to be good p o l y e s t e r p r e c u r s o r s , their r e a c t i o n with alcohols being usually far from quantitative.
In o r d e r to obtain e s t e r linkages, the heterocyclic
amides (azolides) of acrylic and m e t h a c r y l t c acid appear to be m o s t suitable m o n o m e r s (15). It is well known, in fact, that the azolides of carboxylic acids yield e s t e r s quite readily on alcoholysis (17). We have not y e t succeded
to synthesize N - a c r y l o y l - i m t d a z o l e , but
we have already obtained N-methacryloyl imidazole (XIII) (18) CH
I
3
CH2=CI CO I
XIII This m o n o m e r does not homopolymerize well by radical or ionic initiators. However, it m ~ copolymerize with some vinyl m o n o m e r s , and it may prove a valuable synthetic tool for the specific purpose of our investigation. Moreover, we have studied the synthesis and the polymerization of 1 - a cryloyl benzotriazole (XIV) as well as on the exchange ability of the r e s u l t i n g polymer (XV) with alcohols and amines (19) :
10
Pharmacolog/ca/Research Communications, VoL 7, No. 1, 1975
-CH - C H -
CH2=~H
21
CO
CO
I
I
Radical Initiators
X
XW
XV
P o l y - l - a c r y l o y l b e n z o t r i a z o l e is a very good p r e c u r s o r for polyes s t e r s , as well or for polyamides. It r e a c t s smoothly with a number of alcohols and amines under mild conditions, and up to now appears the reagent of choice for the purpose of our investigation. If p o l y - l - a c r y l o y l benzotriazole (XV) is allowed to react with amino alcohols, only the amino groups of the latter react, thus giving hydroxylated p o l y a c r y l a m i d e s , as indicated in the following simple C ase
:
XV
+ H2N-CH2CH2OH
-
>
"-CH2-CHCO i
OH
X
This allow the synthesis of a number of hydroxylated p o l y m e r s which may not be easily obtained by other means. These polymers may be used as m a t r i c e s to which c a r b o x y l - b e a r i n g drugs may be bound by e s t e r linkages. This method has been utilized to obtain nicotinic a c i d - p r e c u r s o r p o l y m e r s , which will be described in a following paper. 3.. Solubil[zing units It may be easily inferred that many h o m o p o l y m e r s of acryloyI derivatives of n o n - m a c r o m o l e c u l a r drugs, obtained by one of the methods discussed above, would be completely insoluble in water or the aqueous environments of the biological s y s t e m s . However, the r e l e a s e of the active substances from such m a c r o m o l e c u l e s is expected to take place by h y d r o l y s i s
Pharmacological Research Communications, Vol. 7, No. 1, 1975"
11
of the e s t e r bond. Hydrophobic p r o p e r t i e s would p r o t e c t the e s t e r groups from hydrolysis.
Consequently, the m a c r o m o l e c u l e s which a r e the object of our
investigations m u s t dissolve o r at l e a s t swell in aqueous environments. This can be achieved by introducing, by c o p o l y m e r i z a t i o n techniques, units d e r i ved from suitable hydrophilic m o n o m e r s . These units should be compatible with living t i s s u e s , non-toxic, and their p h a r m a c o l o g i c a l activity should not i n t e r f e r e with the activity of the active substances to be r e l e a s e d . One of these m o n o m e r s could be N - v i n y l - p y r r o l i d o n e . vious work
From p r e -
(12, $3), it would also s e e m that m o n o m e r s such as N - a c r y l o y l -
- m o r p h o l i n e , (X-v'I), N - a c r y l o y l - 4 - m e t h y l - p i p e r a z i n e (XVII), and 2-vinyl-pyr_i dine N-oxide ( . ~ I I I ) , would be suitable for this purpose. CH2=~H C=O
I
CH 2 :=CH [
CH2=CH
C=O
"~ O
I
I CH 3 X'VI
XVII
XVIII
All these m o n o m e r s appear to be highly hydrophilie and give water-soluble homopolymers. 4. Conclusions F r o m the above c o n s i d e r a t i o n s , the following conclusions may be drawn, at p r e s e n t , on the design of m a c r o m o l e c u I a r d r u g s acting as p r e c u r s o r s of n o n - m a c r o m o l e c u l a r active s u b s t a n c e s : 1) ,4 polyvinylic backbone a p p e a r s to be p r e f e r a b l e , at l e a s t when the m a c r o m o l e c u l a r drug has to be taken orally. 2) When possible, an e s t e r - t y p e of linkage between the active s u b s t a n c e and the m a e r o m o l e c u l a r m a t r i x has to be c o n s i d e r e d first. 3) This is best achieved, in m o s t instances, by r e a c t i r g the active substance, or a derivative, with a m a t r i x containing purposely designed units of suita ble reactivity.
12
Pharmaco/ogica] Research Communications, Vol. 7, No. 1, 1975
4) It is usually n e c e s s a r y that the whole m a c r o m o l e c u l e is w a t e r - s o l u b l e , or al least hydrophillic.
This may be achieved by" introducing hydrophillic
units in the m a c r o m o l e c u l a r matrix. 5) Consequently, the main steps of the synthesis of a m a c r o m o l e c u l a r drug of the type above described are: the synthesis of a suitable coupling m o n o m e r , the synthesis of a suitable solubilizing m o n o m e r , the copolymerization of ~he above m o n o m e r s to give a m a c r o m o l e c u l a r m a t r i x containing both solubilizing and coupling units, and finally the binding of the active substance to the above matrix. The last step maY be performed either directly or ind[ rectly, according to the s t r u c t u r e of the active substance itself.
_Aknowledgement This work was supported, in part, by the Italian National Council of R e s e a r c h e s .
References 1. P. F e r r u t i , Communication to the '"XI C o n g r e s s o della Societ~ Chimica Italiana", P e r u g i a (Italy), Oct. 19-21, 1972. 2. U. F. Gruber, "Blood Replacement", pagg. 138-139, Springer-Verlag, Heidelberg (1964), and re[erences ~herefrom.
3. M.A. Marchisio, C.Sbertoli, G. F a r i n a and P. F e r r u t i , cology 1.2 236 (1970)
Europ. g. P h a r m a
. M. A. Marchisio, T~Longo, P. F e r r u t i and F. Danusso,
Communication to th_e 6 th Congress o f the European Society for Experimental Surgery, ,? 1! Halsinborg (Sweden), April 27-29 (1971); European Surgical R e s e a r c h
3_ 240 (1971) . M. A. Marchisio, T. Longo and P. F e r r u t i ,
Communication to the 7th Cong r e s s of the European Society for Experimental SurgeD,, A m s t e r d a m (The Netherlands), April 12-14 (1972); European S u r g i c a l R e s e a r c h 4 312 (1972)
6. M. A. Marchisio, T. Longo and P. F e r r u t i ,
Experientia 29 93 (1973)
Pharmacological Research Commuuications, VoL 7, No. 1, 1975
13
7. P. F e r r u t i , G. Franchi, N. Polentarutt[, F. Danusso and S. Garattin[; J. Med. Chem. t 6 497 (1973) 8. M. Covello, G. C i a m p a and A. Vittoria, 9. Go Ciampa, A. VitLoria and F. Manna, 10. C. Schuerch, therefrom
Annal[ C h i m i c a (Roma) 57 664 (1967) Annali Chimica (Roma) 59 3 (1969)
Advances in P o l y m e r Science 1.0 172 (1972),
11. H.G. Batz, G. F r a n z m a n n , and H. Ringsdorf,
and r e f e r e n c e s
Makromol. Chem.
17___2227 (1973)
12. G, N a t t a , E. C. Vigliani, F. Danusso, B. P e r n i s , P. F e r r u t i and M. A. M a r c h i s i o Atti Accad. Naz. Lincei 40 11 (1966) 13. M. A. M a r c h i s i o , G. Chiappino, P. F e r r u t i and G. F, Gambini, 101 (1970) 14. F. Danusso and P. Ferrut[,
Polymer
Securitas
55
!I 88 (1970)
15. P. F e r r u t i "Functionalization of P o l y m e r s " in J. A. Moore, "Chemical Reactions on P o l y m e r s " , D. Reidel Publ. Inc., Dordrecht, Holland, pag. 73 (1973) 16. P. F e r r u t i , A . B e t t e l l i and A. F e r 6 , 17. H.A. Staab,
Chem. Ber.
18. P. F e r r u t i and G.Cottica,
Polymer
13. 462 (1972)
9_.00 1320 (1957) J. P o l y m e r Scl. P o l y m e r Chem. Ed., in p r e s s
19. P. F e r r u t i , G. Cottica and A. F e r n , !2 553 (1974)
J. P o l y m e r Sci., P o l y m e r Chem. Ed.,
ACTIVE SUBSTANCES
-
PRELIMINARY
CONSIDERATIONS
Pao!o F e r r u t i - Ist. Chimica Industriale del Politecnico Sez. Chimtca Macromolecolare e Materiali P. za Leonardo da Vinci 32 - 20133 Milano, Italy
Received28 May 1974
Summary: Some principles at the basis of the potential i n t e r e s t of m a c r o m o l e c u l a r drugs are defined.
T h e main lines of a r e s e a r c h on purposely tailored synthetic
high p o l y m e r s acting as p r e c u r s o r s of n o n - m a c r o m o l e c u l a r active substances are given.
The chemical aspects of proper design of these p o l y m e r s a r e discussed
in detail.
The pharmacological application of purposely tailored synthetic high p o l y m e r s is at present a l a r g e l y unexploited field. Nevertheless,
ma
c r o m o l e c u l a r drugs appear to be of g r e a t actual or potential interest. The purpose of this communication is to define some principles at the basis of this interest and to give some p r e l i m i n a r y considerations on a r e s e a r c h line which is actually in p r o g r e s s , the f i r s t results of which will be given in the following papers of this series.
Ist pubblication of a collaborative r e s e a r c h project
with the Institute of
Pharmacology and Pharmacognosy, directed by P r o f e s s o r R. Paoletti
2
Pharmacological Research Communications,, Vol. 7, No, 1, 1975
_Forseea~b.le behavioral peculiaritles of macromolecular dru~ in bioI0gieal environments, .and their subdivision Generally speaking, it may be expected that a macromolecular substance compared to an analogous non-macromolecular one shows some substantial behavioral differences in biological environments. The potential possibilities of original applications of macromole cular drugs are based precisely on these forseeable differences linked to their macromolecular nature
(I)
.
The following outline is indicative on
this point : i) A high polymer injected into a living organism is practically non elimina ble through the usual excretion routes. To be eliminated it must first of all undergo a degradation process which brings it to. an average molecular weight lower than a certain threshold estimated at around 10,000 - 20, 000, as found experimentally, for instance, in the case of polyvinylpyrrolidone (2) The above obviously refers only to polymers injected into the body and not taken by other r o u t e s , for instance, o r a l l y .
In c o n t r a s t , in the l a t t e r c a s e s
the high p o l y m e r s p r e s u m a b l y r e m a i n confined to the digestive t r a c t and do not e n t e r the bloodst~ream u n l e s s they a r e b r o k e n down. 2) A m a c r o m o l e c u l a r substance m a y have a different localization in living o r g a n i s m s c o m p a r e d to a s i m i l a r non m a c r o m o l e c u l a r one.
In fact, as we
shall s e e l a t e r o n , high p o l y m e r s a r e capable of p e n e t r a t i n g into the c e l l s , o r at l e a s t into c e r t a i n types of c e l l s , by a specific m e c h a n i s m . F u r t h e r m o r e , it is always possible to a r r a n g e that high p o l y m e r s acting as p r e c u r s o r s of n o n - m a c r o m o l e c u l a r drugs have a coefficient of distribution between the v a r i o u s p h a s e s - e s p e c i a l l y between the aqueous and the lipid p h a s e - differing from that of the active compounds of which they a r e the p r e c u r s o r s . 3) A multifunctional m a c r o m o l e e u l e is usually much m o r e capable than its non m a c r o m o l e c u l a r homologues of i n t e r a c t i n g with other m a c r o m o l e c u l e s p r e s e n t in biological s y s t e m s ,
with m e m b r a n e s o r other cell s t r u c t u r e s , and also with
foreign s u b s t a n c e s accidentally introduced. Leaving out p o l y m e r u t i l i z e d o r t e s t e d as m a t e r i a l s for the c o n s t r u c tion of artificial p r o s t h e s e s ( p o l y m e r s of b i o m e d i c a l interest) which do not fall
Pharmacologica/ Research Communications, Vo/. 7, No. 1, 1975 within the concept of a drug, for the purposes of the present work m a c r o m o l e cular drugs may be divided into two categories: 1) M a c r o m o l e c u l a r substances which act because they are such and generally are not expected to require a change of structure to exert their action. 2) M a c r o m o l e c u l a r substances which act because they are p r e c u r s o r s of p h a r macologically active n o n - m a c r o m o l e c u l a r drugs. We have already p e r f o r m e d two studies in the field of m a c r o m o l e c u lar drugs, regarding substances belonging to tlm f i r s t group (maeromolecuIar drugs acting because they are such) accerdirg to the above classification. These studies deal with polymers having anti-heparin activity (3 - 6)and other with an t[n-,etastatic action (7). The present a study, however, deals with m a c r o m o l e cular drugs of the second group {precursors of active substances).
Main Purposes of the researc_h.on sy_nthetic highA3olymers acting as p r e c u r s o r s of non m a c r o m o I e c u l a r active substances This I'ine of r e s e a r c h proposes to utilize suitable m a c r o m o l e c u l a r m a t r i c e s as vehicles for pharmacologically active substances. This may be achieved by linking the pharmacologically active substances to convenient m a c r o m o l e c u l a r m a t r i c e s , preferably by means of covalent bonds p o s s e s s i n g limited stability in biological environments. This would lead to gradual and, in many cases, kinetically controlled r e l e a s e of the drug. This is an almost completely new field. Among previous r e f e r e n c e s mention may be made, as examples, to the p r e p a r a t i o n of p o l y m e r s of vinyl e s t e r s of salicylic or s i m i l a r acids
(8, 9), and of some derivatives of natural
polysaccharides with antibiotics and other drugs. (10). Reference has also been made, to be possibility of binding p h a r m a c a to m a c r o m o l e c u l a r m a t r i c e s by Batz, Franzmann, and Ringsdorf (11). The aim of this r e s e a r c h is to achieve one of the following r e s u l t s or even both simultaneously : a) Maintenance in time of suitable bloods levels of drugs which normally have an average life in the body too short to be of true therapeutic interest.
3
Pharmacological Research Communications, Vo/. 7, No. 1, 1975
4
b) P r e f e r e n t i a l localization and subsequent r e l e a s e of the drug at the level
of
given t a r g e t cells or groups of cells. E l e m e n t s exist which indicate that even this second point can be achieved in practice. During a now completed study on p o l y m e r s of antisilico ttgeutc action, we h a v e previously d e m o n s t r a t e d that polym~ers of poly-N-oxide type a r e actively concentrated in vivo at the level of the cells of the r e t i c u l o e n dothelial s y s t e m , by m e a n s of a specific m e c h a n i s m (plnocytosts)
(12, 13).
It
is obviuus that a drug whose site of action is expected, f o r instance; to be at the level of such cells is capabl e, at l e a s t in principle, of being conveyed into these cells by m e a n s of m a c r o m o l e c u l a r m a t r i c e s of the type indicated,
and
subsequently r e l e a s e d . This would lead to obvious advantages as r e g a r d s the effectivene~ss and selectivity of action of the drugs.
,_Chem_ical ~$pects of the d e s i g n of p o l y m e r s acting as p r e c u r s o r s . o f non m a c r p _m01eculax~ active substa:rmes The design of m a c r o m o l e c u l a r drugs acting by slow r e l e a s e of non m a c r o m o l e c u l a r active compounds, i n v o l v e s three main aspects;
1} the type
of the m a i n backbones of the m a c r o m o l e c u l a r c a r r i e r s of the active substances; 2) the chemical nature of the bonds with the active substances;
3) the n a t u r e of
the n o n - a c t i v e units, which, for solubilizing p u r p o s e s m u s t be usually added, e. g . , through copolymerization techniques. These t h r e e aspects will be d i s c u s s e d separately. 1) Types of main backbone Usually the t e r m "main backbone" of a m a c r o m o l e c u l e indicates the p a r t of a m a c r o m o l e c u l e r e m a i n i n g after r e m o v a l of the substituents : for example, in vinyl p o l y m e r s (I)
1 X
Pharmacological Research Communications, Vol. 7, No. 1, 1975 the m a i n b a c k b o n e is CH 2 - CHI
-] X
(if) c l o s e l y r e l a t e d to the polyvinylic backbone is the p o l y v i n y l i d e n i c backbone
(m) CH 2 - C I
I ' 1 X
(m) which m a y be c o n s i d e r e d m e r e l y a p a r t i c u l a r c a s e of the polyvinylicbackbone. F o r the p u r p o s e of o u r investigation, p o l y v i n y l i c or p o l y v i n y l i d e n i c b a c k b o n e s a p p e a r to b e v e r y c o n v e n i e n t for the following r e a s o n s : 1) the b a c k b o n e s t h e m s e l v e s a r e c h e m i c a l l y i n e r t and a r e not e x p e c t e d to r e a c t r e a d i l y with the n o r m a l c o m p o n e n t s of a b i o l o g i c a l e n v i r o n m e n t . 2) M o r e i n f o r m a t i o n is a v a i l a b l e on the s y n t h e s i s of p o l y m e r s o r c o p o I y m e r s of s u c h a s t r u c t u r e than of any o t h e r c l a s s of s y n t h e t i c p o l y m e r s . S e m i s y n t h e t i c p o l y m e r s which m a y be of i n t e r e s t could be a l s o d e r i v e d f r o m n a t u r a l l y o c c u r r i n g m a c r o m o l e c u l e s of p o l y - s a c c h a r i d e s t r u c t u r e , e.g. dextrans. P o l y m e r s with a l e s s c o m m o n type of m a i n b a c k b o n e , p o l y - a m i d o a m i n e s (IV), (V) : O
O li
tl 1 3 ~-CH OH C-N-R -N-C-CH CH,_-N-R -N-
or
0
0
CH CH C - N - R - N - C - C H 2 2 i I R R
(v)
2
CH-N2 12 R
x
5
6
Pharmacological Research Communications, VoL 7, No. 1, 1975
from a synthetic point of view appear well suitable for our investigation. P o l y m e r s of this class have been already investigated by our group as antihepartn (4, 6) and aatimetastatic (7) agents. The synthesis of these polymers can be c a r r i e d out under very mild conditions and in the presence of a very l a r g e number of other chemical functions in the side chains {14}; m o r e o v e r , the resulting polymers may be easily designed to be water or oil-soluble by varyl.ng the nature of the R radicals. However, in these polymers the m a c r o m o l e c u l a r backboz~,: should not be expected to be chemically inert, mainly because of the presence of basic t e r t i a r y amino groups. P o l y m e r s having other types of main backbone, e . g . p o l y e s t e r s , p o l y - a m i d e s , polyurethanes, show a low priority for our purpose, at least initially, mainly because the c h e m i c a l r e a c t i o n s involved in the synthesis of these polymers are not easily c a r r i e d out in the presence of reactive functions in the side chains. An important point is, if the main backbone of a hypotheti cal m a c r o m o l e c u l a r drug will be degraded in a biological environment. Most polysaccharides, such as dextrane, are metabolized in m a m m a l i a n organisms. P o l y - a m i d e - a m i n e s , on the other hand, are easily degraded in any aqueous environment even at neutral pH, by hydrolysis of the amide bonds, a reaction probably catalyz ed i n t r a m o l e e u l a r l y by the t e r t i a r y amino groups
(14}.
A poly-vinylic (or-vinylidenic) backbone, on the contrary, should be little, or not at all degraded in the body environment, whatever is the fate of the side-substituents.
This fact is best exemplified by the case of high molecu-
lar w e i g h t poly-N-vinylpyrrolidone, which r e m a i n s indefinitely in the body, without being eliminated, since no degradation occurs which brings the polymer below the renal threshold (2). On the other hand, this behaviour does not elicit obvious toxic side effects in man.
F o r m a c r o m o l e c u l a r drugs to be taken orally, the non-degradability of the main backbone is obviously of advantage, since only the active substance which is r e l e a s e d is absorbed. On the c o n t r a r y , if the m a c r o m o l e c u l a r drug has to be used,
for
Pharmacological Research Communications, Voi. 7, No. 7, 1975
7
instance, p a r e n t e r a l l y its main backbone m u s t be d e g r a d a b l e , in o r d e r to
avoid any long term permanence and possibly any accumulatlon in the body of macromolecular residues.
2) Binding of d r u g s t.o a m a c r o m o ! e c u l a l : back.b0ne Covalent bonds of l i m i t e d stability in biological e n v i r o n m e n t s a r e in our opinion the m o s t suitable ones for attaching a n o n - m a c r o m o l e c u l a r acti ve s u b s t a n c e to a m a c r o m o l e c u l a r backbone, when a slow and gradual r e l e a s e of this active subst,,mce in the body should be achieved. E s t e r bonds should be c o n s i d e r e d f i r s t , d r u g m o l e c u l e allows such a type of binding.
when the s t r u c t u r e of the
The e s t e r bonds can be m a d e m e
r e o r l e s s e a s i l y cleaved by v a r y i r g the s t r u c t u r e of the side groups in the m_a c r o m o l e c u l a r m a t r i c e . This
will be e x e m p l i f i e d in a following p a p e r of this
s e r i e s . O t h e r types of linkage, such as amidic or imino bonds, m a y also be suitable in p a r t i c u l a r c a s e s . The e s t e r bond may be utilized when the d r u g m o l e c u l e c o n t a i n s hydroxyl group or c a r b o x y l groups. In the f o r m e r c a s e , a way to obtain polym e r s o r c o p o l y m e r s containing the d e s i r e d d r u g - p r e c u r s o r units (V1) might be the s y n t h e s i s and the p o l y m e r i z a t i o n or the c o p o l y m e r i z a t i o n of, for instan_ ce, the a c r y l i c o r m e t h a c r y l i c e s t e r s of the d r u g i t s e l f (VII): -
HC1
AOH + C H 2 = CH-COC~
~ CH 2 = CH-COO-A
-CH2-CHI C~
I
(or o t h e r synthetic r e a c t i o n s )
VII
O
X
f
A
A = Drug r e s i d u e
VI
Simple as it m a y s e e m , this method has s e v e r a l d i s a d v a n t a g e s . F i r s t , other groups p r e s e n t in the drug m o l e c u l e m a y s e r i o u s l y i n t e r f e r e with the p o l y m e r i z a t i o n p r o c e s s .
Secondly, it is n e c e s s a r y to s y n t h e s i z e
and p o l y m e r i z e a new m o n o m e r i c d e r i v a t i v e for e v e r y active s u b s t a n c e
to
be bound to a m a e r o m o l e c u l a r backbone. S i m i l a r c o n s i d e r a t i o n s hold t r u e also when d r u g s containing amino groups a r e to be bound to a m a c r o m o l e c u Iar m a t r i c e with amidic bonds.
Pharmacological Research Communications, Vol. 7, No. 1, 1975
8
A much b e t t e r method (15)~'involves the synthesis of m o n o m e r s bearing
in the side chains suitable chemical functions, yielding the d e s i r e d
e s t e r (or amido) linkages. P o l y m e r s or c o p o l y m e r s obtained from these m o n o m e r s (VIII) could give directly the d e s i r e d end products (VI, IX) by t r e a t m e n t with the active substances to be bound.
.4-OH A-NH2
-oH2-I!:-o,
-CH2-CH-
+
CO I
B
X
A - drug residue B = leaving group r e s i d u e This method p r e s e n t s two significant advantages: first,, no i n t e r actions a r e expected to occur with many chemical functions which may be p r e s e n t in the drug molecule, as, for instance, with u n s a t u r a t e d s t r u c t u r e s ; secondly, a single " m o t h e r " p o l y m e r m~w be utilized to bind covalently a n u m b e r of active s u b s t a n c e s , provided they b e a r free hydroxyl or amino grouis, simply by t r e a t i n g this p o l y m e r with the active s u b s t a n c e s . Acryloyl and m e t h a c r y l o y l c h l o r i d e s do not appear a p p r o p r i a t e m o n o m e r s for this p u r p o s e , mainly b e c ~ s e of their high r e a c t i v i t y and lack of selectivity. Consequently, a study on other active derivatives of acrylic o r m e t h a c r y l i c acids is presently in p r o g r e s s . We have already obtained high p o l y m e r s of acrylic (X) and m e t h a c r y l i c (XI) e s t e r s of N-hydroxy-succ_i nimide (16):
CH2=~H CO f
0
X
?H 3 CH =C 2
CO
XI
Pharmacological Research Communications, Vol. 7, No. 1, 1975
9
The resulting p o l y m e r s a r e very good p r e c u r s o r s of p o l i a c r y l a m t des or polymethaerylamtdes, as indicated in the following synthesis of l i n e a r p o l y - N - a l l y l a c r y l a m i d e , which cannot be otherwise p r e p a r e d : N
m
-CII2-CHI
radtcal X initiators
a.Uylamine
-CH-CH21 CO
-C4115NO 3
NH
CO I
O O I
I
I
O
CH2-CH=CH 2 X
X
.
_
J
Other activated e s t e r of acrylic and m e t h a c r y l i c acids have been subsequently described (11).
The above p o l y m e r s , however, did not prove
to be good p o l y e s t e r p r e c u r s o r s , their r e a c t i o n with alcohols being usually far from quantitative.
In o r d e r to obtain e s t e r linkages, the heterocyclic
amides (azolides) of acrylic and m e t h a c r y l t c acid appear to be m o s t suitable m o n o m e r s (15). It is well known, in fact, that the azolides of carboxylic acids yield e s t e r s quite readily on alcoholysis (17). We have not y e t succeded
to synthesize N - a c r y l o y l - i m t d a z o l e , but
we have already obtained N-methacryloyl imidazole (XIII) (18) CH
I
3
CH2=CI CO I
XIII This m o n o m e r does not homopolymerize well by radical or ionic initiators. However, it m ~ copolymerize with some vinyl m o n o m e r s , and it may prove a valuable synthetic tool for the specific purpose of our investigation. Moreover, we have studied the synthesis and the polymerization of 1 - a cryloyl benzotriazole (XIV) as well as on the exchange ability of the r e s u l t i n g polymer (XV) with alcohols and amines (19) :
10
Pharmacolog/ca/Research Communications, VoL 7, No. 1, 1975
-CH - C H -
CH2=~H
21
CO
CO
I
I
Radical Initiators
X
XW
XV
P o l y - l - a c r y l o y l b e n z o t r i a z o l e is a very good p r e c u r s o r for polyes s t e r s , as well or for polyamides. It r e a c t s smoothly with a number of alcohols and amines under mild conditions, and up to now appears the reagent of choice for the purpose of our investigation. If p o l y - l - a c r y l o y l benzotriazole (XV) is allowed to react with amino alcohols, only the amino groups of the latter react, thus giving hydroxylated p o l y a c r y l a m i d e s , as indicated in the following simple C ase
:
XV
+ H2N-CH2CH2OH
-
>
"-CH2-CHCO i
OH
X
This allow the synthesis of a number of hydroxylated p o l y m e r s which may not be easily obtained by other means. These polymers may be used as m a t r i c e s to which c a r b o x y l - b e a r i n g drugs may be bound by e s t e r linkages. This method has been utilized to obtain nicotinic a c i d - p r e c u r s o r p o l y m e r s , which will be described in a following paper. 3.. Solubil[zing units It may be easily inferred that many h o m o p o l y m e r s of acryloyI derivatives of n o n - m a c r o m o l e c u l a r drugs, obtained by one of the methods discussed above, would be completely insoluble in water or the aqueous environments of the biological s y s t e m s . However, the r e l e a s e of the active substances from such m a c r o m o l e c u l e s is expected to take place by h y d r o l y s i s
Pharmacological Research Communications, Vol. 7, No. 1, 1975"
11
of the e s t e r bond. Hydrophobic p r o p e r t i e s would p r o t e c t the e s t e r groups from hydrolysis.
Consequently, the m a c r o m o l e c u l e s which a r e the object of our
investigations m u s t dissolve o r at l e a s t swell in aqueous environments. This can be achieved by introducing, by c o p o l y m e r i z a t i o n techniques, units d e r i ved from suitable hydrophilic m o n o m e r s . These units should be compatible with living t i s s u e s , non-toxic, and their p h a r m a c o l o g i c a l activity should not i n t e r f e r e with the activity of the active substances to be r e l e a s e d . One of these m o n o m e r s could be N - v i n y l - p y r r o l i d o n e . vious work
From p r e -
(12, $3), it would also s e e m that m o n o m e r s such as N - a c r y l o y l -
- m o r p h o l i n e , (X-v'I), N - a c r y l o y l - 4 - m e t h y l - p i p e r a z i n e (XVII), and 2-vinyl-pyr_i dine N-oxide ( . ~ I I I ) , would be suitable for this purpose. CH2=~H C=O
I
CH 2 :=CH [
CH2=CH
C=O
"~ O
I
I CH 3 X'VI
XVII
XVIII
All these m o n o m e r s appear to be highly hydrophilie and give water-soluble homopolymers. 4. Conclusions F r o m the above c o n s i d e r a t i o n s , the following conclusions may be drawn, at p r e s e n t , on the design of m a c r o m o l e c u I a r d r u g s acting as p r e c u r s o r s of n o n - m a c r o m o l e c u l a r active s u b s t a n c e s : 1) ,4 polyvinylic backbone a p p e a r s to be p r e f e r a b l e , at l e a s t when the m a c r o m o l e c u l a r drug has to be taken orally. 2) When possible, an e s t e r - t y p e of linkage between the active s u b s t a n c e and the m a e r o m o l e c u l a r m a t r i x has to be c o n s i d e r e d first. 3) This is best achieved, in m o s t instances, by r e a c t i r g the active substance, or a derivative, with a m a t r i x containing purposely designed units of suita ble reactivity.
12
Pharmaco/ogica] Research Communications, Vol. 7, No. 1, 1975
4) It is usually n e c e s s a r y that the whole m a c r o m o l e c u l e is w a t e r - s o l u b l e , or al least hydrophillic.
This may be achieved by" introducing hydrophillic
units in the m a c r o m o l e c u l a r matrix. 5) Consequently, the main steps of the synthesis of a m a c r o m o l e c u l a r drug of the type above described are: the synthesis of a suitable coupling m o n o m e r , the synthesis of a suitable solubilizing m o n o m e r , the copolymerization of ~he above m o n o m e r s to give a m a c r o m o l e c u l a r m a t r i x containing both solubilizing and coupling units, and finally the binding of the active substance to the above matrix. The last step maY be performed either directly or ind[ rectly, according to the s t r u c t u r e of the active substance itself.
_Aknowledgement This work was supported, in part, by the Italian National Council of R e s e a r c h e s .
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