**16. Conclusions and future directions**

**15. In vitro intraconverion of Paracetamol ProD 1 to the parent drug**

The hydrolysis of paracetamol **ProD 1** was studied in four different media; 1N HCl, buffer pH 3, buffer pH 6.6 and buffer pH 7.4. The prodrug hydrolysis was monitored using HPLC analysis. At constant pH and temperature the release of paracetamol from its prodrug was followed and showed a first order kinetics. *kobs* **(h-1)** and t1/2 values for the intraconversion of paracetamol **ProD 1** was calculated from regression equation obtained from plotting log concentration of residual of paracetamol **ProD 1** vs. time. The kinetics results in the different

> **Medium** *kobs (h-1) t½ (h)* 1N HCl No reaction No reaction

Buffer pH 3 6.3 x 10-5 3 Buffer pH 7.4 6.1 x 10-4 0.3

As shown in Table 4 the hydrolysis rate of paracetamol **ProD 1** at pH 7.4 was the fastest among all media, followed by pH 6.6 medium. In 1N HCl no conversion of the prodrug to the parent

**Table 4.** The observed *k* value and *t*1/2 of paracetamol **ProD 1** In 1N HCl and buffers pH 3and 7.4.

**Figure 18.** First order hydrolysis plot of paracetamol **ProD 1** in (a) buffer pH 3 and (b) buffer pH 7.4.

At pH 7.4 and 6.6 paracetamol **ProD 1** is mainly exists as the carboxylate anion form which is expected to undergo fast hydrolysis according to Bruice's mechanism shown in Figure 16. At pH 3, the prodrug exists in both form, the carboxylate anion and the carboxylic free acid forms since the pKa of the prodrug is about 3. In 1N HCl, the prodrug is entirely exists as the

**paracetamol**

432 Application of Nanotechnology in Drug Delivery

dug was observed.

media are summarized in Table 4 and Figure 18.

The quantum mechanics (QM) calculations in different methods revealed that the acidcatalyzed hydrolysis efficiency of processes **34**-**42**, atenolol **ProD 1**-**ProD 2**,amoxicillin **ProD1** and cephalexin **ProD 1** is significantly sensitive to the pattern of substitution on the carboncarbon double bond and nature of the amine leaving group. The linear correlation found between the reaction rate and strain energy difference between the intermediate and the reactant (Es INT-GM) supports the notion that the reaction is governed by strain effects. Furthermore, the linear correlation of the calculated DFT and experimental EM values reinforce the credibility of using DFT methods for energy and rate predictions for the kind of processes reported in this section.

Comparisons of the calculated DFT properties for processes **34**-**40**andatenolol prodrugs **ProD1**-**ProD2** with the calculated DFT properties for the acid-catalyzed hydrolysis of acyclovir prodrugs and cefuroxime (Figure 19) demonstrate that while for processes **34**-**40** and atenolol prodrugs **ProD 1**-**ProD 2,** the rate-limiting step was the collapse of the tetrahedral intermediate in the processes of cefuroxime prodrugs and acyclovir prodrugs the rate-limiting step was the tetrahedral intermediate formation. This is might attributed to the nature of the amine leaving group involved in the tetrahedral intermediate collapse step. **<sup>H</sup>**

**R1, R2 = H, CH3**

**Figure 19.** Chemical structures for acyclovir and cefuroxime prodrugs.

Comparison of the calculated t 1/2 value (63.2 hours) for atenolol **ProD 1** to the experimental value (3.82 hours) indicates that while the B3LYP/6-31G (d,p) value is overestimated (about 17 times larger than the experimental) the one obtained by mpwpw91/6-31+G(d,p) was much more closer (6.3 hours). The discrepancy between the calculated B3LYP/6-31G (d,p) and experimental values might be attributed to (i) B3LYP/6-31 G(d,p) is a DFT method without dispersion corrections and (ii) PCM solvation model (calculations in presence of solvent) is not capable of handling calculations in acidic aqueous solvent.

**Author details**

Rafik Karaman1,2\*

Palestine

**References**

1-5.

Address all correspondence to: dr\_karaman@yahoo.com

2 Department of Science, University of Basilicata, Potenza, Italy

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1 Bioorganic Chemistry Department, Faculty of Pharmacy, Al-Quds University, Jerusalem,

Prodrugs for Masking the Bitter Taste of Drugs

http://dx.doi.org/10.5772/58404

435

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The experimental t 1/2 value for atenolol **ProD 1** at pH 5 was 133 hours and at pH 7.4 no hydrolysis was observed. The lack of the hydrolysis at the latter pH might be due to the fact that at this pH atenolol **ProD 1** exists mainly in the ionized form (*pKa* about 3-4). As mentioned before the free acid form is a mandatory requirement for the acid-catalyzed hydrolysis to proceed.

In a similar manner to that observed in the intraconversion of atenolol **ProD 1, the** acidcatalyzed hydrolysis of both, amoxicillin **ProD 1**and cephalexin **ProD 1** was much faster in 1N HCl than in pH 2.5 and 5 (Figures13 and 14). At 1N HCl the t ½ values for the intraconversion of amoxicillin **ProD 1**and cephalexin **ProD 1**was in both cases about2.5hours. On the other hand, in pH 7.4, both amoxicillin **ProD 1**and cephalexin **ProD 1** were entirely stable and no intraconversion to the parent drugs was detected. The salient points emerged from our study on Bruice's system are as follows: (i) the cyclization rate of Bruice's system was found to be dependent on the difference in the strain energies of the intermediate and reactant, and no relationship was found between the reaction rate and the distance between the nucleophile and the electrophile. (ii) The reactions of strained di-carboxylic semi-esters are more efficient than the less strained ones, and the reactivity extent was linearly correlated with the strain energy difference between the intermediate and reactant. (iii) The activation energy required to give a stable transition state for a strained di-carboxylic semi-ester is less than that for the unstrained semi-ester, since the conformational change from the reactant to the transition state in the former is smaller, and (iv) based on the linearity found between the relative rate, the activation energy and the difference in strain energies of the intermediate and reactant for Bruice's di-carboxylic semi-esters we have proposed two paracetamol prodrugs, which were synthesized and their in vitro kinetics was studied. Future strategy to achieve more efficient atenolol prodrugs capable of increasing the liquid formulation stability, eliminating atenolol bitterness and releasing the parent drug in a programmable manner is synthesis of atenolol prodrugs having *pKa* around 6 (intestine pH). At the pH of the intestine the planned prodrugs will exist mainly in the acidic form which has the capability to undergo an acid-catalyzed hydrolysis to provide the active drug, atenolol.
