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January 1998 Volume 1, Number 3-4, Section 4

Double Protease Combinations
   A Primer on a Drug’s Levels in Blood or Pharmacokinetics

As you know the first protease inhibitors were approved just about two years ago. We are just beginning to explore some of the different ways they can be used. Combining a protease inhibitor with a NNRTI has begun to be explored but additional research is ongoing and is needed to better understand the effects of combining a given protease inhibitor with a particular NNRTI. Ritonavir+ saquinavir was the first double protease combination to be explored. Studies of a number of other double protease combinations are ongoing or will be evaluated. By this time next year we should have a better understanding of the potency, tolerability and safety of these combinations. We should have a better understanding of how to use them and in which circumstances they may be most useful. Will they be useful against protease resistant virus? Will they be more potent than a single protease inhibitor? Under which circumstances could 2 be preferable to 1? Raising blood levels of a protease drug by combining it with a NNRTI or other protease may help people who fail protease therapy because they had inadequate blood levels for that drug. These are questions research is addressing.

Primer on a Drug’s Levels in Blood or Pharmacokinetics. In the articles in this section, the NNRTI section, and other parts of the newsletter you will see references to the pharmacokinetics (PK) of a drug. The PK of a drug refers to the concentration or levels of a drug in your blood. This explanation should help you better understand the discussions. The AUC, Cmin, and Cmax are referred to as the pharmacokinetic parameters or measures of drug levels in the blood. They are considered an important factor in a drug’s antiviral potency and durability. The Cmin, which is also called the trough, is the lowest blood level of a drug during a fixed dosing period (for example, 8 hours for indinavir, or 12 hours for ritonavir); it is generally believed that the trough level for a drug should be above a certain level to adequately suppress virus replication. If the trough is too low, resistance may begin to develop. There may be a difference of opinion about the importance of the Cmin in certain circumstances. The AUC (area under the curve) is the total exposure of a drug in your blood over a fixed dosing period (e.g., 8 or 12 hours). The Cmax is the highest or peak level achieved by the drug in your blood over the fixed dosing period. The significance of Cmax is uncertain. Some experts believe a higher Cmax can be associated with more severity of side effects and that a lower Cmax may be associated with less severity, if the toxicity is drug concentration dependent. Some experts say that a higher Cmax may be beneficial to penetration of virus reservoirs and have additional benefits related to antiviral effects. Both may be true.

Indinavir (IDV)+ Nelfinavir (NLF). Researchers reported preliminary or interim analysis of 1000mg indinavir (IDV) q12 + 750 nelfinavir (NLF) q12 (every 12 hrs) saying that the trough level of nelfinavir at 12 hours was too low. They are concerned about longer term efficacy and resistance. So they are planning to explore the combination using higher doses of nelfinavir. It is uncertain how much of an increase in nelfinavir dose will be necessary to raise the trough to a level where their concerns are alleviated. As you raise the dose of any drug, potential side effects and toxicities are a concern. But in the study of NLF bid dosing reported by Dr Michael Sension, the 1250 mg bid dose appeared to be tolerable in that small short term study. See results in the article on "Double Protease Combinations".

Investigators reported that coadministration of IDV 1000mg every 12 hours with 750mg NLF every 12 hours resulted in Cmax, AUC and trough (blood levels) similar to IDV at 800 every 8 hours without NLF. But IDV had no obvious effect on NFV steady state blood levels resulting in low trough levels. They also concluded that the combination was generally well tolerated and had a substantial antiviral effect.

At week 8, 7/10 study participants were <500 copies/ml; 9/10 were <2,500 copies/ml; for 9/10, the CD4 increase was +156 cells. 1 patient discontinued due to rash. Other adverse events were: diarrhea/loose stools (6), bloating (2), nephrolithiasis (1). By visual observation of the graph in the abstract of the individual viral load reductions for the evaluable study participants it appeared as if all but one person at week 8 achieved a reduction of between -1.7 to -2 log from baseline. But again, the primary concern is the prevention of resistance and the durability of the antiviral effect.

The judgement of the researchers is that higher doses of NLF need to be explored to assure durability.

Indinavir+Ritonavir. In this study, investigators explored different dosing combinations of the two drugs in healthy individuals to see the effect on different measures of blood levels or pharmacokinetics (AUC, Cmax, and Cmin or trough). The drugs were taken under non-fasting conditions (370 kcal, 5% fat) and every 12 hours. The optimal doses have not been defined. Further studies are planned which if successful should produce guidelines for using the combination.

Study results showed that ritonavir can substantially increase indinavir AUC, Cmax, Cmin and half-life. Investigators reported that "across groups, ritonavir increased indinavir AUC up to 480%, and Cmax up to 110%. The mean apparent halflife was increased from about 2 hours to 5.8 hours." They found no effect of indinavir on ritonavir blood levels.

Investigators explored 4 different combination regimens in this study and said, if given every 12 hours they will likely give higher trough, lower Cmax and similar renal clearance of indinavir compared with the standard dose of 800 mg every 8 hours. But each dose regimen had varying effects on indinavir blood levels. There will be a number of considerations in determining an effective, tolerable and safe dosing regimen. Therefore, it is premature to experiment using these drugs together until further studies have been conducted.

Investigators concluded all regimens were well tolerated overall, and all adverse events were minor. They reported that mild circumoral parasthesia, a tingling or numbness around the mouth which can occur as a side effect to ritonavir, diminished with time, and that no new side effects were observed.

Nelfinavir+Saquinavir. Two studies preliminarily address the potential use of this combination. The first explores the pharmacokinetics (PK) or interaction between the two drugs when they are used together. 14 individuals enrolled in study who were protease inhibitor naive and were either stable on nucleosides, washed out from the nucleosides they were on or in the case of 1 person was nucleoside naive.

Investigators said that "multiple doses of SGC SQV 1200 mg did not have an effect on nelfinavir PK. Multiple doses of nelfinavir, however, increased SQV single dose mean AUC by 4.9 times and Cmax by 2.8 times. An 800 mg tid dose of SQV in combination with NFV 750 mg tid provides similar exposure (drug levels) to the 1,200 mg dose given alone. The SQV PK is non-linear, which was factored into the decision to decrease SQV dose in combination dosing by only 33% rather than 4 fold. Steady state blood levels of SQV SGC at 800 mg tid and NFV 750 tid in combination at 4 weeks were virtually identical to levels of each at 2 weeks (at 2 weeks SQV SGC was given at 1200 mg tid) when they were given alone." See Table 7

Table 7. Nelfinavir+Saquinavir: Median Viral Load and CD4 Changes from Baseline
11/14 had prior nucleoside experience and 3 were treatment naive.


1 mo


4 mo


6 mo


11 mo


CD4 increase 327









Viral load decrease 39,917

-2.25 log


-2.1 log


-2 log


-2.3 log


%<500 copies /ml  









Baseline CD4 range was 19 to 621 cells. Baseline viral load was 39,917 copies/ml and the range was from 19,847 to 109,064 copies/ml.

* The %<500 at 11 months is based upon only 3 patients and may not be predictive of the actual % when more individuals are evaluated.

The second trial is called the SPICE study and compared 4 treatment regimens:

• nelfinavir+saquinavir
• nelfinavir+saquinavir + 2 nucleosides
• saquinavir + 2 nucleosides
• nelfinavir + 2 nucleosides

The new formulation of saquinavir called Fortovase was used in this study. The dosing regimen used was 750 mg nelfinavir tid and 800 mg saquinavir tid. This 48 week study was not designed to compare the efficacy of the 4 regimens. It was designed to explore pharmacokinetics and preliminary antiviral activity. Some study participants were nucleoside naive and some were nucleoside experienced. The number of naive and experienced were not necessarily evenly divided between the 4 treatment arms, which is why you cannot compare the CD4 increases and viral load decreases between the 4 arms. However, the efficacy and safety data of two regimens nelfinavir+saquinavir+2 nucleoside regimen and nelfinavir+saquinavir are useful. Data is extended to only 16 weeks, so it is preliminary.

53% of participants in this arm were treatment naive. 25% who were treatment experienced switched to 2 new nucleosides. The baseline CD4 and viral load for this group was 300 and about 50,000 copies/ml. See Table 8

Table 8. Nelfinavir+Saquinavir

NFV+SQV+2 nukes

week 16

treatment exp, %< 400 copies/ml

75% (n=20)

treatment naive, %<400 copies/ml*

90% (n=23)

group as a whole, %<400 copies/ml


group as a whole, <50 copies, Ultrasensitive test


VL decrease , Amplicor 400 copy test*

-1.95 log

VL decrease , Ultrasensitive 50 copy test*

-2.57 log (n=43)

CD4 increase


* This is not an intent-to-treat analysis. It includes only individuals who remained on study treatment. If a person withdrew because of side effects, no antiviral response, etc. they were not included in this data. If they were included the percentage would likely be lower.

Reported side effects considered at least possibly related or of unknown relationship to study drug and moderate, severe or life threatening in degree included: diarrhea (35%), nausea (8%), abdominal pain (2%), vomiting (4%), asthenia (4%), arthalgia (2%). There were 2 discontinuations for adverse events or intercurrent illnesses.

For the 2-drugregimenofnelfinavir+ saquinavir taken without nucleosides baseline CD4 and viral load were 301 and about 63,000 copies/ml; 56% were treatment naive. See Table 9

Table 9. Nelfinavir + Saquinavir


Week 16

treatment exp, % <400 copies/ml

55% (n=21)

treatment naive, %<400 copies/ml*

55% (n=26)

group as a whole, <50 copies, Ultrasensitive test


VL decrease, Amplicor 400 copy test*

-1.58 log

VL decrease, Ultrasensitive 50 copy test*

-1.96 log (n=45)

CD4 increase


Side effects included: diarrhea (46%), abdominal pain (7%), vomiting (2%), fatigue (6%), asthenia (2%), arthalgia (2%). 2 individuals discontinued study treatment.

ABT-378 + Ritonavir. ABT-378 is a second generation protease inhibitor in an early stage of development. In preclinical studies, ABT-378 has shown it was significantly more potent then ritonavir, has a different resistance profile, has an ability to suppress ritonavir resistant virus, and significantly less than standard doses of ritonavir greatly enhanced the blood drug levels of ABT-378. Abbott Labs, the developer of ABT-378, has reported that the side effects so far seen have been favorable compared to those seen for ritonavir at this stage of its development.

The first clinical trial in HIV infected humans started in November ‘97. Based upon information emerging from this study, it is expected that additional studies will begin in early ‘98 including exploration of the potential for the combination of ABT-378/ritonavir in individuals for whom prior protease therapy has failed.

The pattern for the development of resistance to ABT-378 was studied in vitro and reported at ICAAC. As occurs with other protease inhibitors, mutations appeared in a sequential or step wise way with increasing concentrations of ABT-378 : I84V, L10F, M46I, T91S, V32I, I47V. Selection at the final concentration of ABT-378 resulted in a secondary mutation of V47A, and a reversion at 32 back to wild-type. You will note the potentially significant absence of a mutation at position 82. A mutation at this position is key to ritonavir and indinavir resistance. Based upon the in vitro cross-resistance data reported at ICAAC, it appears as though ABT-378/ritonavir may not be able to suppress all levels of ritonavir resistance. High level ritonavir resistance may not be able to be suppressed by ABT-378/ritonavir. However, lower levels extended to possibly 10-20 fold resistance or a little higher may be able to be suppressed because of the potency and antiviral activity expected due to the high drug level concentrations that appear achievable in the study described in the following paragraph (see commentary below).

Abbott researchers reported the effects (pharmacokinetics) of combining varying single doses of ABT-378 and ritonavir in both a non-fasting and fasting state to measure the effect of food. 12 different combinations of the two drugs were explored using doses of ABT-378 ranging from 100 to 800 mg, and using ritonavir in doses ranging from 50 to 300 mg. The AUC (drug blood levels) of ABT-378 was increased 100-300 fold when taken with ritonavir, and blood levels achieved were significantly higher than the IC50 for wild-type virus. The IC50 is the inhibitory concentration necessary to suppress 50% of virus replication. It is a standard research method for testing antiviral potency of a drug. It did not appear as though ABT-378 significantly altered ritonavir blood levels.

Investigators concluded that with single-dose coadministration of the two drugs, no subject withdrew due to an adverse event; 378/ritonavir was generally well-tolerated; one subject taking the dosing combination of 800 mg 378 and 200 mg ritonavir had a grade 2 elevation of SGOT/SGPT (liver function tests) and one subject receiving placebo had a grade 2 SGOT elevation. Food did not effect the pharmacokinetics of the combination; and, the high levels of drug concentration achieved can be expected to be highly suppressive of HIV. As stated above, several dose combinations are being explored in treatment naive HIV infected individuals including 200 mg 378+100 mg ritonavir and 400 mg 378+100 mg ritonavir in a twice daily dosing regimen which will include d4T+3TC. The PK of once daily dosing is still being researched by Abbott.

Ritonavir+Nelfinavir. Preliminarily, it appears as though ritonavir increases nelfinavir blood levels about 2 to 2.5 fold. Two small studies are under way exploring two different bid regimens: 750 nelfinavir bid + 400 bid ritonavir; 1000 nelfinavir bid + 400 mg bid ritonavir. Data is expected to be reported in February 1998 at the Human Retroviruses Conference. Again, it is premature to experiment with these combinations until more information is available.

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