Introduction the most important excipients in in DDS

Introduction

Controlled drug delivery systems that are considered to
deliver drugs at predetermined rates for predefined periods of time, have been
used to control the limitations of conventional drug formulations. In some
cases drug has to be delivered in response to ph in the body, Infact it would
be advantageous if the drug could be administered in a manner that precisely
matches the physiological needs at proper times at the specified target sites.
The range of fluids in various sections in the GIT may provide environmental
stimuli that are responsive to drug release ph. Stimuli-responsive polymers are
one of the most important excipients in in DDS and pharmaceutical formulations.
These are designed to produce specific and desired PH concentration activated
response according to body physiological environment variations.

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PH sensitive drug delivery systems (PSDDS) deliver the drug
at specific time as per the pathophysiological need of the body and gives
improved patient compliance and therapeutic efficacy that is why it is gaining
importance.  

Polymers

All the Ph sensitive
polymers consist of pendant acidic (carboxylic acid and sulfonic acids) and
basic (ammonium salts) groups that either accept or release protons in response
to changes in environment PH. The polymers having large number of ionizable
groups are called polyelectrolytes

The charge density of the polymers is dependent on the PH
and ionic concentration of the outer solution (in which the polymer is
introduced). Swelling or de-swelling of the polymer can be caused by altering
the ph of the solution.

1.      
Poly-acidic polymers are unswollen at the low ph
as the acidic groups will be protonated and thus unionized.

2.      
With increasing ph polyacidic polymers are going
to swell

3.      
In polybasic polymers with decreasing ph
ionization of basic group is going to increase

4.       Derivatives
of acrylic acid are most commonly used ph sensitive polymers.

Methodologies for PH
Sensitive Drug Delivery

Properties of PH
Sensitive Hydrogel

Hydrogels comprises of cross linked polyelectrolytes that
have big differences in swelling properties depending upon the environmental PH.
The pendant acidic or basic groups on polyelectrolytes undergo ionization
however it is difficult due to electrostatic effects exerted by other adjacent
ionized groups, this makes the apparent dissociation constant (ka) different
from that of corresponding monoacid or monobase. ionizable groups presence  on polymer chains results in swelling of the
hydrogels. The swelling of the polyelectrolyte hydrogels happens due to the
electrostatic repulsion among charges that are present on the polymer chain ,
the degree of swelling can be influenced by any condition that reduce
electrostatic repulsion such as ph, ionic strength and type of counter ions.
The swelling and ph responsiveness of polyelectrolyte hydrogels can be balanced
by using the neutral comonomers such as 2-hydroxyethyl methacrylate and methyl
methacrylate.

Different comonomers provide different hydrophobicity to the
polymer chain, as a result different ph sensitive behaviour is shown.

Example

Hydrogels made up of
poly methacrylic acid grafted with poly ethylene glycol have unique Ph
sensitive properties. the acidic protons of carboxylic acid of PMA at low PH
interact with ether oxygen of PEG through hydrogen bonding resulting in
shrinkage of hydrogels. At high PH the carboxylic groups of PMA become ionized,
the resulting complexation results in swelling of the hydrogels.

Applications of ph
sensitive hydrogels

Controlled drug
delivery

PH Sensitive
hydrogels have been most frequently used to develop controlled release
formulations for oral administration. The ph in stomach (